GEOL 1107, Lab #1
INTRODUCTION TO ROCKS AND MINERALS
INTRODUCTION
There are many different rocks to be found in the Earth. To simplify their
description all rocks are classified as one of three types: 1) igneous; 2)
sedimentary; and 3) metamorphic. All rocks are composed of many different
types of minerals. Minerals are the fundamental components or basic
“building blocks” of all rocks and as such are very important in
understanding the science of geology. The ability to recognize and classify
common minerals and the rocks they make up is an essential part of this
introductory geology course.
A comprehensive understanding of minerals and rocks, and how to recognize
them, is important because it helps when reconstructing the geological
history of the solar system, planet Earth and all the events, processes, features
and life forms that have evolved since the Earth formed. Understanding rock
and mineral identification is also important because it plays an essential role
in the interpretation of the various conditions (physical and chemical) that
affect rock and mineral formation. For example, it may be vital to know the
mineral content of a particular igneous granite-type rock so that its
relationship with other, economically important, minerals such as gold, may
be recognized and understood. Information about granite formation can be
obtained by examining the size, shape and relative abundance of various
minerals such as quartz, feldspar and hornblende. Geologists are always
trying to identify rocks and minerals; it is an acquired skill that comes with
practice.
There are thousands of different minerals. However, the minerals you will
become familiar with in the following series of labs are the most common
types and are the dominant rock forming minerals. In subsequent labs, the
majority of rocks you will see are the common types made up of a small
number of rock-forming minerals.
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GEOL 1107, Lab #1
LEARNING OBJECTIVES
The objectives of the lab portion of this introductory geology class are:
1. Recognize, compare and apply mineral and rock properties,
2. Recognize and classify common rock forming minerals,
3. Recognize and classify intrusive and extrusive igneous rocks,
4. Recognize and classify sedimentary rocks,
5. Recognize and classify metamorphic rocks.
TERMINOLOGY
Before continuing, we need to clearly understand the meaning of a few
fundamental terms or concepts:
1. A mineral is a naturally occurring, inorganic substance composed of
a specific ratio of chemical elements arranged in a re-occurring
crystalline structure.
2. All minerals are made up of atoms. Chemical composition of a
mineral refers to the type of atoms and the ratio of atoms in the
atomic structure. In a particular mineral it is not practical to actually
see or count atoms. Instead, the ratio of atoms is presented to better
understand the chemical composition. For example, the mineral
quartz is SiO2 which means it is made up of one silicon (Si) atom for
every two oxygen (O) atoms. In any given mineral the chemical
composition is either constant or ranges within narrowly defined
limits.
3. A crystal or crystalline substance is a solid material with a well
defined, ordered arrangement of atoms that are unique to that
substance. All minerals are crystals because they all have a reoccurring crystalline structure, however, most of the time it’s too
small to see.
4. A rock is a naturally occurring aggregate of one or more minerals.
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GEOL 1107, Lab #1
LABORATORY 1
PROPERTIES OF MINERALS
The objective of lab 1 is to recognize the properties found in all minerals.
These are then used to begin identifying minerals. While there are
approximately 5400 known minerals, you will learn only 45 of the common
minerals. To do this, you must first learn to observe and evaluate their
physical properties. In this first lab you will take a systematic look at the
physical properties of a mineral that are most useful for identification and
classification purposes.
The 12 most important physical properties of a mineral, and the ones you will
use for identification purposes are:
1.
2.
3.
4.
5.
6.
Hardness
Luster
Streak
Colour
Diaphaneity
Crystal Habit
7. Tenacity
8. Reaction with acid
9. Density and Specific Gravity
10. Magnetism
11. “Special Properties”
12. Cleavage
THIS IS REALLY IMPORTANT
READ THIS AND SAVE YOURSELF SOME TROUBLE LATER.
A really important thing to remember about all minerals is that they vary.
What this means is that some minerals will have a very specific property
that make them easy to identify, such as the emerald-green colour of
malachite. Other minerals will have a number of different properties that
also make it easy to identify them. The mineral plagioclase feldspar has a
white colour, thin straight lines or striations on its cleavage surface and a
blue - green iridescence. Many minerals have a property or properties that
are very useful for identifying them, but because they vary you may not see
them in all samples of the same mineral. So how then do you identify a
mineral? It’s a simple process. Identification is based not on one but as
many properties as possible. Also make sure you look at more than one
sample of the same mineral so you can see as many properties as possible
and how they might look different. List all the properties you see and
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GEOL 1107, Lab #1
make a decision, chances are you’ll be right. At the end of the day, the
name you give to a mineral is based on the properties you see and the sum
total of what they tell you.
Now let’s begin.
PHYSICAL PROPERTIES OF MINERALS
1. HARDNESS
All minerals are hard, some are just harder than others. The hardness of a
mineral is a measure of a minerals resistance to abrasion or scratching when
minerals are scratched against one another or when certain tools are used to
scratch it. The hardness of a mineral is dictated by its chemical composition
and crystal structure, the very things that define a mineral.
You can test the hardness of a mineral by measuring the “scratchability” of a
sample while referring to a scale of minerals of known hardness. Ten reference
minerals are arranged in a standard hardness scale known as the Mohs Scale
of Hardness which lists them in order of increasing hardness (Table 1 - 1).
Talc, number 1 on the scale (Figure 1 - 1), is the softest and most easily
scratched mineral while diamond, at number 10, is the hardest and most
difficult to scratch (Figure 1 - 2). Each mineral will scratch the one below it in
the scale but not the one above it. For example, number 4, the mineral fluorite,
will scratch number 3, calcite, but not 5, the mineral apatite. When you have an
unknown mineral scratch it with the minerals in Mohs Scale of Hardness or one
of the standard tools to determine a value for hardness.
To aid in identification, common objects or tools of known hardness - a
copper penny, glass plate, steel nail, streak plate - are included in Mohs scale
of hardness to help determine the hardness of a mineral. A streak plate is a
small, flat piece of white, unglazed porcelain with a hardness of 6.5. A glass
plate and steel nail each have a hardness of approximately 5.5. These three
tools should scratch fluorite (hardness 4) but not quartz (hardness 7). Your
fingernails have a hardness of 2.5. You should be able to scratch the mineral
gypsum with your fingernail. In turn, if you were to scratch the mineral
feldspar or quartz with your fingernail your fingernail would be abraded,
scratched or worn down instead of the mineral.
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Table 1 - 1. Mohs Scale of Hardness
Hardness Value
Mineral
1 (softest)
Talc
2
Gypsum
(2.5 fingernail)
3
Calcite
(3.5 copper penny)
4
Fluorite
5
Apatite
(5.5 glass plate, steel nail)
6
Feldspar
(6.5 streak plate)
7
Quartz
8
Topaz
9
Corundum
10 (hardest)
Diamond
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Scale is in centimeters
Figure 1 - 1. The mineral talc, a white mineral or light coloured with a hardness of 1.
Figure 1 - 2. The mineral diamond, at 10 on the hardness scale.
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As with all mineral properties described in this manual there are points, hints
or bits of extra information you might find useful. Here are some ‘hints’ that
have to do with using hardness to help identify a mineral.
1. When testing for hardness you should try different approaches: test one
mineral against another, test with one tool (i.e. steel nail) and then another
(i.e. glass plate). Testing with a steel nail and glass plate helps determine
whether the mineral in question is in the softer (1 - 5) or harder (6 - 10)
part of the scale. Sometimes only an approximate or a range of values is
possible.
2. When testing one mineral against another, scratch one mineral with the
other, and then reverse the procedure. Minerals with the same or nearly
the same hardness will scratch each other.
3. When using the glass plate or streak plate, hold them flat on the table and
draw the sample firmly across them. NEVER try to use a glass or streak
plate to scratch a mineral sample, as it may splinter in your hand.
4. Check to see if you have made a scratch by probing with the tip of a steel
nail or your fingernail. Clean the sample of any powder. A fine scratch
may be evident in the sample. For a closer look take a picture with your
phone and zoom in for a closer look. In a lab use a microscope.
5. Sometimes, hardness will vary when samples of the same mineral are
examined. For example, the hardness of hematite ranges from about 1 to 6
depending on the crystalline structure of the sample.
6. A sample may contain a mixture of minerals, so you must be careful to
clearly identify what mineral or part of a mineral you are testing.
7. Some minerals are composed of small aggregates of granular crystals that
easily break apart. Thus, the mineral may appear to be softer than the true
hardness of the individual crystals. You are in fact not testing the mineral
hardness but the hardness of the aggregate. The minerals olivine and
hematite are examples of minerals that may behave like this. Look closely
to determine what it is you are attempting to scratch and what the results
are.
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8. All mineral hardness tests should be made on a fresh mineral surface.
Often a weathered mineral surface has an outer coating composed of some
other softer mineral such as iron oxide, also known as rust. These
coatings may mask the true hardness of the mineral.
9. Different parts or surfaces of the same mineral sample may have different
hardnesses. This is related to the crystalline structure of the mineral.
Therefore, when in doubt, scratch the mineral on more than one surface
when determining hardness.
EXERCISE 1 - 1: DETERMINING MINERAL HARDNESS
Obtain a set of “hardness minerals” that make up the Mohs scale. Arrange
them in order of increasing hardness from 1 through 9 (diamond is not
present for obvious reasons). Be sure to make use of all of the “tools” at your
disposal (fingernail, penny, glass, steel nail, streak plate). As you proceed,
take note of the hints for hardness described in this lab.
This is how the hardness of a mineral is used to help identify a mineral. You
compare the hardness of one mineral to another, or to a tool such as your
fingernails, a glass plate or a nail file to come up with a measure of hardness.
Often the hardness number you assign to a mineral is an approximation. It is
your best estimate and just one of several properties used to identify a
mineral.
2. LUSTER
A minerals luster is a measure of the amount and quality of light reflected
from its surface. There are two basic types of mineral luster: metallic and
non-metallic.
Metallic luster is highly reflective. It looks like polished metal such as
aluminum foil, a gold ring or a brass doorknob. Figure 1 - 3 shows the
metallic luster of the mineral pyrite. Non-metallic minerals do not look like a
piece of metal, they have a range of different lusters including glassy and
earthy. Figure 1 - 4 shows the glassy luster of quartz (A) and the earthy luster
of hematite (B). Remember it’s not the colour that determines luster, it’s the
way light reflects off the surface of the mineral. All the common mineral
lusters are listed in table 1 - 2. Use this table as a reference when we finally
start mineral identification.
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GEOL 1107, Lab #1
Figure 1 - 3. The metallic luster of the mineral pyrite
Figure 1 – 4A. The non-metallic glassy luster of the mineral quartz (A).
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Figure 1 – 4B. The non-metallic earthy luster of the mineral hematite (B)
A few minerals have a luster somewhere between metallic and non-metallic.
These minerals are sub-metallic, and they, fortunately, are relatively rare.
For the purpose of this lab, we describe sub-metallic minerals, such as
sphalerite, as metallic because they look more metallic than non-metallic and
we can simplify mineral identification.
Table 1 - 2. Mineral Luster.
Images of all the example minerals in this table are found in tables 4 - 1 and 4 - 2.
Luster
A. METALLIC
Appearance
Example
shiny, grey, highly reflective
galena, graphite
molybdenite, hematite
generally dull grey, with many
shiny, high reflective surfaces
magnetite
shiny, yellow/brass/gold,
highly reflective
pyrite, pyrrhotite
chalcopyrite
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GEOL 1107, Lab #1
B. SUB-METALLIC
shiny but not highly
reflective frequently dull
sphalerite,
C. NON-METALLIC
Non-metallic lusters include a variety of common sub-types.
i) ADAMANTINE
has a high degree
of sparkle
diamond
ii) VITREOUS
or GLASSY
looks like broken glass or quartz, fluorite
clear quartz
olivine
iii) RESINOUS
looks like a semitransparent resin or glue
microcrystalline quartz
iv) GREASY
typically has vitreous
luster along with
greasy feel or oily shine
talc
graphite, halite
v) PEARLY
“soft” shiny appearance
gypsum, dolomite,
calcite
vi) EARTHY
dull appearance
limonite, bauxite
Remember, minerals will vary. The luster of a mineral may be different
depending on the quality of the sample. All you can do is look at as many
samples as possible and make a decision. Also, remember, mineral
identification is based on more than one property and a number of samples
that together describe a single mineral type.
3. STREAK
Streak is the colour of a mineral when it has been crushed into a powder.
Streak is determined by scratching the mineral across the surface of streak
plate. Certain minerals may have a range of different colours but will always
have a consistent, diagnostic streak colour. Quartz comes in a variety of
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GEOL 1107, Lab #1
colours - white, clear, purple - but always has a white streak. Most nonmetallic minerals, especially the white or light coloured ones such as calcite,
talc, gypsum, and fluorite, have white or very pale steaks that all look the
same. As result streak is often of limited use for identifying these types of
minerals. But it’s still a visible property and every bit of information helps.
Streak is more useful when identifying metallic minerals than non-metallic
minerals. Metallic minerals often have a dark streak similar to the colour of
the mineral. Examples of some common metallic minerals and the colour of
their streaks are listed in table 1 - 3.
Table 1 - 3. Metallic minerals and their streak
Metallic Mineral
Streak
Pyrite
greenish – black streak
Magnetite
black streak
Hematite
reddish – brown streak
Limonite
yellow-brown streak
Graphite
blue – grey streak
EXERCISE 1 - 2: STREAK
Compare the streaks of the dark mineral samples in your boxes of common
and economic minerals (#60, #124, #126, #78 and #151). Note which dark
minerals have dark streaks, which ones have lighter colored streaks, which
have a metallic luster, and which have a non-metallic luster.
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GEOL 1107, Lab #1
Sample Number
Streak
Luster
#60
#124
#126
#78
#151
Figure 1 - 5. Bright, shiny metallic silver gray hematite (A)
and dull, earthy red hematite (B).
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Scale is in centimeters
Figure 1 - 6. Colour of streak common to both types of hematite in figure 1 - 5.
The mineral hematite has two distinct colours because of the different ways it
forms. Coarse crystalline hematite is a bright, shiny metallic bluish gray
colour (Figure 1 – 5 A). The other common form of hematite is the fine
crystalline, dull, earthy red variety (Figure 1 – 5 B). Figure 1 – 6 shows the
streak that is common to both these two types of hematite.
Now examine the hematite samples (#74 and #75) in the box of common
minerals. The range of colors is due to the form in which the mineral
hematite occurs. Coarse crystalline hematite is a bright, shiny metallic bluish
gray. The other common form of hematite is the fine crystalline, dull, earthy
red variety. Scratch these two samples (#74 and #75) across a streak plate and
note the streak.
What is the color of the streak from the two samples?
Is streak a useful diagnostic tool for hematite? Explain your answer.
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Here’s a bit of useful information to think about for streak. If you have a
mineral sample and a streak plate, put the streak plate on a flat surface and
scratch the mineral across the plate and observe the colour of the powder. What
colour is the streak and where did it come from?
What if the mineral you’re trying to identify is harder than the streak plate
which is 6.5 on Mohs Scale of Hardness? What if the mineral is quartz which
has a hardness of 7? Will a powder appear on the streak plate and if so, what
does it mean? Likely what you’ve done is scratched the streak plate and the
powder is from the streak place, not the mineral. For minerals harder than
6.5 another way needs to be found to crush a mineral into powder. This is
one reason why geologists carry rock hammers.
4. COLOUR
The colour of a mineral is produced when light strikes its surface. The light
that’s reflected, the light that you see, is what creates the colour you see. It is
the most important feature for mineral identification. Colour may be of
limited use when identifying some minerals, mainly because many minerals
have a variety of colours.
Take the minerals cinnabar (Figure 1 - 7 A) and quartz (Figure 1 - 7 B).
Cinnabar has a very bright scarlet red colour which is very useful for
identification purposes. Quartz, on the other hand, occurs in a variety of
different colours, including pink (rose quartz), yellow, gold, green, blue violet (amethyst) and dark gray or black (smokey quartz). As far as quartz is
concerned, colour is by itself, not a great diagnostic tool. Other properties are
more useful when examining quartz.
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GEOL 1107, Lab #1
Figure 1 - 7. The bright red colour and earthy luster of cinnabar (A) and the pink
colour and glassy luster of rose quartz (B). The most common white colour of quartz
is shown in figure 1 - 4 A.
Many different minerals will have the same colour, which also makes mineral
identification more complex than it need be. For example, both quartz and
fluorite have purple varieties. One of the best ways to distinguish fluorite from
quartz is cleavage and crystal habit, mineral properties we’ll deal with later.
Similarly, the minerals hornblende and pyroxene commonly have black to dark
greenish black colours, dark streaks, and comparable values for hardness.
Despite their similar features, when hornblende and pyroxene are placed sideby-side their difference in colour becomes more obvious, hornblende is black
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GEOL 1107, Lab #1
(Figure 1 - 8 A) and pyroxene is greenish black (Figure 1 - 8 B).
Figure 1 - 8. The colours of hornblende (A) and pyroxene (B). Hornblende is black
while pyroxene has a greenish black colour, a useful diagnostic property.
When using colour as a property for mineral identification keep the following points
in mind.
1. Numerous minerals, such as pyroxene and hornblende, have dark colours
such as dark gray or black because they have an abundance of iron
(denoted by the formula Fe in its chemical composition) in their crystal
lattice.
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GEOL 1107, Lab #1
2. Many minerals are not pure, meaning that they have been contaminated or
mixed with other atoms or minerals. They may also be present in such
small quantities that they are not easily observed. Under these
circumstances the use of colour as a diagnostic tool must be done very
carefully and with the use of a microscope, hand lens or a close up look at
an image.
EXERCISE 1 - 3: THE COLOUR OF MINERALS
Examine the minerals listed below and note their chemical formula. Observe
how color changes are reflected by changes in chemical composition, a simple
enough comparison given that the composition of a mineral will directly
influence the type and amount of light reflected from its surface.
Mineral
Sample
Number
Chemical Formula
Muscovite
#106
KAl2(AlSi3)O10(OH)2
Biotite
#24
K(Mg,Fe)3(Al,Fe)Si3O10
(OH,F)2
Olivine
#114a
Mg2SiO4
Hornblende
#78
(Ca,Na)2-3
(Mg,Fe+2,Fe+3,Al)5
(Al,Si)8O22(OH)2
Pyrite
#124
FeS2
Sphalerite
#153
(Zn,Fe)S
Color
How do you account for the color differences between these different mineral samples?
Why are biotite and hornblende different in colour compared to muscovite and
quartz?
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Now examine the large quartz and calcite samples in the lab. For both
minerals, the various colors are due to the presence of very minute amounts
of impurities. Both quartz and calcite are colorless or clear in pure form.
Answer these questions by filling in the box below.
What is the most common color of quartz?
What is the most common color for calcite?
Mineral Chemical Composition
Quartz
SiO2
Calcite
CaCO3
Common Color
Is color a useful diagnostic tool for identifying quartz and calcite? Why? Why not?
5. DIAPHANEITY
Diaphaneity is the ability of a mineral to transmit light. The three types of
diaphaneity related to minerals are:
1. Opaque minerals such as galena or pyrite that completely block the
passage of light through them.
2. Translucent minerals, such as feldspar and hornblende, which allow
some light through. If these samples are held against a light source,
the rims may allow a little dull light through them.
3. Transparent minerals such as quartz, muscovite or calcite allow light to
pass through.
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GEOL 1107, Lab #1
These three terms are rather imprecise, and often of limited use when
identifying a mineral, because the quality of a mineral may vary and with it
our ability to see through it. For example, “transparent” minerals such as
quartz are not always very clear. However, at the very least you should be
able to see the shape and size of another object through a mineral like
muscovite.
When considering diaphaneity as a mineral property for identification, here
are points to remember.
1. The ability of a mineral sample to transmit light depends, in part, on its
thickness. Thin sheets of muscovite are transparent while some of the
larger, thicker samples vary from translucent to opaque.
2. Diaphaneity also depends upon mineral purity. Quartz, for example, in
pure, crystalline form is transparent. It easily transmits light. However,
as you have seen in the section on colour, quartz also has a variety of
common colours because of the presence of impurities. Therefore quartz
may be opaque, translucent or transparent. Diaphaneity as a diagnostic
property is generally of limited use in mineral identification.
6. CRYSTAL HABIT
All minerals have a crystalline structure or habit. This is a reoccurring
arrangement of chemical atoms that define the characteristic form, shape or
combination of forms and shapes of a mineral.
Depending on the types of atoms in the mineral and the ratio of atoms
different crystal shapes will form. Figures 1 - 9, 1 - 10 and 1 - 11 illustrate the
crystal habit of the minerals quartz, calcite and pyrite, three of the common
minerals introduced in this lab.
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GEOL 1107, Lab #1
Figure 1 - 9. The six-sided, hexagonal crystal habit of a quartz crystal.
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Figure 1 - 10. The six-sided, rhombohedral crystal habit of calcite.
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GEOL 1107, Lab #1
Figure 1 - 11. The six-sided cubic crystal habit of pyrite.
All three minerals in figures 1 - 9, 1 - 10 and 1 - 11 have 6 sides. What makes
them different is that each has a different angle between adjacent sides.
EXERCISE 1 - 4. CRYSTAL HABIT
The quartz crystals in figures 1 - 9 and 1 - 12 have the same chemical formula,
they are all composed of the same ratio of silicon and oxygen (SiO2) that
makes up all quartz crystals. A quick look at all these crystals indicates they
all have the same general shape, very much like the line diagram of a quartz
crystal in figure 1 - 9. However, the crystals in figure 1 - 12 are not all
identical. Some are longer, some are wider. There is some variation in the
reoccurring crystalline structure.
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Scale is in centimeters
Side 2
Side 1
Side 3
Side 6
Side 4
Side 5
Figure 1 - 12. Quartz crystals. Image A shows a cluster of quartz crystals of
different sizes that all grew together. Image B shows a single quartz crystal viewed
from the end or top with its six-sided hexagonal shape.
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Now examine at least two quartz crystals (sample #133) provided in the lab
and in the box of common minerals. Note their similarities and differences.
Use figure 1 – 12 to help with your observations.
What can you conclude about the size of the crystals? Are they all the same size?
What can you conclude about their shape and form? Are they similar in any way?
How many faces or sides does each crystal have?
Now use the goniometer – a small protractor-like device - provided in the lab
to measure the crystal faces of one of the crystals. Figure 1 – 13 illustrates a
goniometer and how to use it. See your instructor for more help. A crystal
face is the flat, smooth, shiny surface that appears on the crystal. Some
crystals have only a few crystal faces while others may have many crystal
faces. For at least two quartz crystals measure the angle between each pair of
adjacent faces. Make a note of what the angles are.
Figure 1 – 13. A goniometer used to measure mineral crystals. It consists of a
protractor with a 0o to 180o scale, a rotating ruler to measure angles and a scale to
measure the width and length of crystals.
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GEOL 1107, Lab #1
Perhaps you will agree that the best explanation for the regular nature of
quartz crystals, as demonstrated by the equal number of crystal faces and the
consistent angles between adjacent crystal faces, is the very regular,
reoccurring internal crystalline structure of the quartz mineral.
Crystal Texture
In nature a mineral may be present in crystalline form, as fragments of
crystals, or as aggregates composed of numerous whole or partially formed
crystals.
Mineral aggregates consist of numerous individual crystals packed together
in a lump, or solid mass. Each crystal in an aggregate will have a
characteristic habit, but often times will not show it because there was no
room for the faces or flat, shiny surfaces of a crystal to grow and appear as the
minerals formed. A hand lens, microscope or close up of an image will help
you to see crystals when they are quite small or only partially formed.
Recognition of crystal habit and individual crystal sides or faces requires an
understanding of cleavage. Sometimes when a mineral is broken, a flat shiny
surface appears, very much like the face of a crystal. This, however, is
cleavage and cleavage is very different from crystal habit. More about
cleavage later in lab 2.
IMPORTANT
Crystal habit is how a mineral grows. Cleavage is how a mineral breaks.
For the common minerals introduced in this lab there are only two that
routinely display crystal habit: quartz (Figure 1 - 9 and Figure 1 - 12), calcite
(Figure 1 – 10) and pyrite (Figure 1 - 11). For most of the remaining minerals
in this lab a well-developed crystal habit is rare and so, as a diagnostic tool, is
of limited use.
There are a number mineral textures that form as a result of the crystal habit
of a mineral. These are used to help identify many different minerals. Crystal
texture refers to the size, aspect or orientation of a crystal or aggregate of
crystals. Crystal habit is about the shape of individual crystals. Table 1 - 4
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below describes and illustrates a number of common crystal textures.
Table 1 - 4. The crystal structure of minerals.
A full description of the minerals in this table and their crystal texture are
found in tables 4 - 1 and 4 - 2 in lab 4.
1. Granular refers to a mineral with many, large crystals visible to the
unaided eye. They often appear as a densely packed mass of crystals.
A good example is olivine (Figure 1 - 14).
Scale is in centimeters
Figure 1 - 14. The granular crystalline texture of olivine.
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2. Aphanitic describes a mineral with crystals that are too small to see,
even with a close-up image, a microscope or a hand lens. Dolomite
(Figure 1 - 15) is a mineral with an aphanitic crystal texture.
Scale is in centimeters
Figure 1 - 15. The aphanitic or fine-grained texture of dolomite.
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3. Massive implies a homogenous or non-directed aspect or orientation of
the minerals in a rock. All the crystals, regardless of size, appear
randomly oriented. Bornite has a massive crystal texture (Figure 1 - 16).
Scale is in centimeters
Figure 1 - 16. The massive crystalline texture of bornite.
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4. Foliate describes a parallel to subparallel arrangement of flat or
tabular crystals. The mineral biotite (Figure 1 - 17) has a flat sided,
tabular crystal texture.
Scale is in centimeters
Figure 1 - 17. The foliate crystalline texture of biotite. In this image, many flat,
platey crystals are laid one on top of the other like the pages of a book.
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5. Dense or Compact indicates tightly interlocking, usually coarser or
large size crystals, giving a coherent or “tough” appearance or texture
to the mineral or mineral aggregate. Magnetite (Figure 1 - 18) has a
dense or compact crystal texture. This crystal texture may appear
similar to a granular texture.
Scale is in centimeters
Figure 1 - 18. The dense, compact crystalline texture of magnetite.
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GEOL 1107, Lab #1
6. Earthy implies an aphanitic or very fine crystalline texture composed
of loosely-held, soft mineral crystals. Limonite (Figure 1 - 19) has an
earthy crystal texture.
Scale is in centimeters
Figure 1 - 19. The earthy crystalline texture of limonite.
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GEOL 1107, Lab #1
7. Microcrystalline refers to an aphanitic crystalline texture with
tightlyinterlocking crystals so that the mineral or mineral aggregate
has a hard, coherent texture. A good example is microcrystalline
quartz (Figure 1 - 20).
Scale is in centimeters
Figure 1 - 20. The microcrystalline texture of microcrystalline quartz.
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GEOL 1107, Lab #1
8. Coliform or Reniform texture refers to fine-grained crystal
aggregates with characteristic rounded, kidney-shaped or “bubbly”
looking surfaces. These are formed by a build up of numerous thin
concentric shells or layers of a mineral as it precipitates. The mineral
hematite (Figure 1 - 21) is the only common mineral in this lab with a
distinctive coliform or reniform crystal texture.
Scale bar is 5 millimeters
Figure 1 - 21. Close up of the coliform or reniform crystalline texture of hematite.
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GEOL 1107, Lab #1
9. Oolitic and Pisolitic textures refer to round or ovoid pellets that have
a concentrically layered internal structure. Oolitic pellets are smaller
than 2 mm. Pisolitic pellets are bigger than 2 mm. The mineral
bauxite (Figure 1 - 22) has a well-defined pisolitic crystal texture.
Pisolitic
Pellet
Oolitic
Pellet
Scale is in centimeters
Figure 1 - 22. The oolitic and pisolitic crystalline textures of bauxite.
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GEOL 1107, Lab #1
10. Drusy texture is a layer or layers of fine or aphanitic crystals that
form a coating on other crystals or rock surfaces. Drusy crystals
may also be found infilling a cavity in a rock sample. Cinnabar
(Figure 1 - 23) is a good example of a mineral with a drusy
crystalline texture.
Scale is in centimeters
Figure 1 - 23. The drusy crystalline texture of cinnabar. This red mineral
appears as coating on a light gray-brown mineral.
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GEOL 1107, Lab #1
11. The following terms describe the habit or general shape of individual
crystalline grains in an aggregate of minerals.
Tabular is a shape like a piece of paper such as individual biotite
and muscovite crystals (Figure 1 - 24). Prismatic with numerous,
flat reflective surfaces is what a quartz crystal looks like (Figure 1 25). Bladed occurs in a flat, elongated mineral such as kyanite
(Figure 1 - 26). Acicular or needle-shaped is the crystal structure of
the mineral tremolite (Figure 1 - 27).
Scale is in centimeters
Figure 1 - 24. Individual crystals of muscovite and biotite.
The light-coloured muscovite (left mineral) is propped up against the
black biotite (right mineral) with each showing its tabular shape. When
many muscovite or biotite minerals are stacked together, they form the
foliate texture in figure 1 - 17.
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GEOL 1107, Lab #1
Scale is in centimeters
Figure 1 - 25. Prismatic quartz crystal.
Scale is in centimeters
Figure 1 - 26. Flat, elongated bladed crystals of kyanite.
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GEOL 1107, Lab #1
Scale is in centimeters
Figure 1- 27. Acicular, or needle-like, crystals of the mineral tremolite.
12. Non - crystalline refers to minerals that have no crystalline
structure. These types of minerals form so fast, often when lava
cools and solidifies underwater, that there is little or no time for
crystals to form. A prime example is the igneous rock obsidian
(Figure 1 - 28) which has no crystalline structure.
Figure 1 - 28. The non-crystalline texture of obsidian, an igneous intrusive rock.
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GEOL 1107, Lab #1
7. TENACITY
The ability of a mineral to resist breaking, crushing, bending or tearing is
known as tenacity. The most often happens when a mineral is struck by a
hard object such as a rock hammer. The following terms in table 1 - 5
describe the tenacity of minerals.
Table 1 - 5. Types of mineral tenacity.
Tenacity
Description
Mineral
i) Brittle
Breaks or powders easily
quartz
fluorite
calcite
ii) Flexible
Bends but does not resume its
vermiculite
shape when pressure is released
iii) Elastic
Bends but resumes its shape
when pressure is released
biotite
muscovite
iv) Malleable
Can be hammered out into
thin sheets
gold and silver
v) Sectile
Can be cut into thin shavings
with a knife
talc
graphite
vi) Ductile
Can be drawn into a wire
copper
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GEOL 1107, Lab #1
8. REACTION WITH ACID
The carbonate minerals calcite and dolomite can be distinguished from other
minerals because of the way they react with acid. The chemical reaction can
be observed when a drop of cold, dilute hydrochloric acid (10% HCl) is
placed on one of these two minerals. Calcite (CaCO3) will react immediately
and strongly with 10% HCl. The visible reaction you see is the effervescence
or release of carbon dioxide (CO2) bubbles produced by the chemical reaction
that dissolves the mineral. The reaction between dolomite (Ca,Mg(CO3)2) and
10% HCl, on the other hand, is much slower and is usually only observed
when the dolomite mineral has been crushed into powder.
The reaction of hydrochloric acid and these two carbonate minerals, calcite
and dolomite, is a very important diagnostic tool. Calcite and dolomite are
two very common rock forming minerals and their rapid identification is both
easy and important in the study of minerals and rocks.
EXERCISE 1 - 5: ACID TEST TO IDENTIFY CALCITE AND DOLOMITE
Place a drop of 10% HCl on the mineral calcite (sample #26a) and note what
happens. The formula for the chemical reaction between calcite (CaCO3) and
hydrochloric acid (HCl) is:
CaCO3 (s) + 2HCl (aq)
➔
CaCl2 (aq) + CO2 (g) + H2O
Note the immediate and rapid release of carbon dioxide (CO2) bubbles.
Now place a drop of 10% HCl on the mineral dolomite (sample #52) and note
what happens. Is there any result? Scratch the dolomite to produce a little
powder on the sample and try another drop of acid (observe closely). Now is
there any reaction? The formula for the chemical reaction between
hydrochloric acid and dolomite is:
CaMg(CO3)2(s) + 4HCl(aq) ➔ Ca+2(aq) + Mg+2(aq) + 4Cl-1(aq) + 2CO2(aq) + 2H2O
The 10% HCl reacts with both carbonate minerals. However, the strength of
the reaction, as indicated by the number of bubbles of carbon dioxide and the
rate at which they are released, is a standard method for distinguishing
between calcite and dolomite.
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GEOL 1107, Lab #1
IMPORTANT NOTE
GO EASY ON THE ACID - A SMALL DROP WILL DO! Although the
acid is very dilute, please handle it carefully. It the acid spills on your skin,
it will not burn but you should wash the area as a precaution. If acid comes
in contact with the eye, it will sting and you should wash out your eye with
water. It will make holes in clothing that usually don’t appear until after
the next laundering. Always blot off excess acid with paper towels after
testing.
9. DENSITY and SPECIFIC GRAVITY
The density of any substance is the measure of its mass per unit volume and
can be expressed in grams per cubic centimeter (g/cm3). The specific gravity
(S.G.) is the ratio of the mass of a substance to the mass of an equal volume of
water. The density and specific gravity of water is 1 g/cm3. The mineral
galena has a density of 7.5 g/cm3 and a specific gravity of 7.5.
Opaque, metallic minerals such as galena (Figure 1 - 29) and molybdenite
(Figure 1 - 30) tend to be much denser than non-metallic minerals such as
potassium feldspar or K – feldspar (Figure 1 - 31) which have a density of 2.6
g/cm3. Native metals including copper, silver and gold have the highest
densities or specific gravity of all known minerals. For example, native gold
(Figure 1 - 32) has a density of 19.3 g/cm3. In a river, high density, heavier
pieces of gold is transported and deposited differently compared to lighter,
less dense pieces or grains of quartz. This is an important factor when
searching for gold nuggets in river deposits.
Of the non-metallic minerals, barite (Figure 1 - 33) has one of the highest
densities for a non-metallic mineral at 4.5 g/cm3, which can be a very
important diagnostic property.
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GEOL 1107, Lab #1
Figure 1 - 29. The metallic mineral galena has a density of 7.5 g/cm3 .
Figure 1 - 30. The metallic mineral molybdenite has a density of 4.7 g/cm3 .
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GEOL 1107, Lab #1
Figure 1 - 31. The non-metallic mineral potassium feldspar has a density of 2.6 g/cm3 .
Figure 1 - 32. The metallic mineral gold has a density of 19.3 g/cm3 .
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GEOL 1107, Lab #1
Figure 1 - 33. The non-metallic mineral barite has a density of 4.5 g/cm3 .
EXERCISE 1- 6: DENSITY OF MINERALS
Three metallic minerals have the following properties.
Name
Density
Colour
Chemical Formula
Graphite
2.2 g/cm3
Gray
C
Molybdenite
4.7 g/cm3
Blueish Gray
MoS2
Galena
7.5 g/cm3
Gray
PbS
All three of these minerals are similar in appearance yet they have very different
densities. Using each minerals chemical formula what is it about these minerals that
gives them such different densities?
(Hint: C is carbon, Mo is molybdenum, Pb is lead)
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GEOL 1107, Lab #1
All three of these minerals are similar in appearance. All are gray in colour
with a metallic luster. Samples of each mineral are provided in the lab.
Based on what you know about density and specific gravity determine which sample
is graphite and which is molybdenite? Write the correct sample numbers in the
spaces below.
Mineral
Sample Number
Graphite
Molybdenite
Galena
10. MAGNETISM
A few metallic minerals, such as magnetite and pyrrhotite, are strongly
attracted to a magnet, which means they are magnetic. Other minerals may
look like they could be magnetic, they appear similar to magnetite, but they
aren’t. Magnetite and galena, for example, are both dark metallic minerals,
however, magnetite is strongly magnetic whereas galena is not magnetic.
Figure 1- 34 shows a small bar magnet stuck to the side of the mineral
magnetite. Similarly, pyrrhotite and pyrite are yellow, metallic minerals.
However, pyrrhotite is magnetic while pyrite is not. While the presence of
iron (Fe) in the mineral is key to its magnetic properties, it does not guarantee
the mineral is magnetic. Pyrite for example contains iron but is not magnetic.
The magnetic properties of a mineral can best be determined by placing a
small bar magnet on the mineral. If the magnet sticks to the mineral it is
magnetic. If it does not it is non-magnetic. Similarly, small bar magnets can
be suspended near the mineral. If the magnet swings towards the mineral it
is magnetic. If it does not it is non-magnetic.
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GEOL 1107, Lab #1
Bar magnet
Scale is in centimeters
Figure 1 - 34. A small bar magnet attracted to the side of magnetite.
As with all mineral properties there is an important point to remember for
magnetite, especially because you are using the small bar magnet.
The magnetic property of a mineral may be difficult to determine if it is not
present in pure form. Small inclusions of magnetite in a rock may give a false
indication of magnetism. Similarly, if magnetite and pyrrhotite are present in
only small quantities in a rock then its magnetic properties may go
undetected.
EXERCISE 1 - 7: MAGNETIC PROPERTIES
Some of the more common magnetic minerals are of economic significance.
This is because the iron in their crystal structure is a valuable resource as well
as the source of magnetism.
Which minerals in the box of economic mineral are magnetic?
List below all the minerals, with their sample numbers, you find to be magnetic.
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GEOL 1107, Lab #1
11. “ SPECIAL PROPERTIES ”
Some minerals have additional, perhaps unusual properties that may be very
useful for making a positive identification. These include:
1. Odour
Kaolinite, a clay mineral (sample #83), is common in sediments and soils.
It often produces a distinctive “earthy“ odor when slightly wet.
Sphalerite, a sulphide mineral (sample #151), emits a characteristic
sulphurous or “rotten egg” smell when dilute HCl is placed on it.
2. Feel
Some minerals have a characteristically greasy feel to them. Some, such as
graphite (sample #64) and talc (sample #161) derive this particular
property from the fact that they are relatively soft (both have a hardness of
1) and have perfect cleavages in one direction. When the surfaces of these
minerals are rubbed with your finger a greasy or slippery feel results
because small mineral particles are easily dislodged along cleavage
surfaces.
Others minerals, such as halite (sample #73) feel greasy because when
they are rubbed the moisture on your hand partially dissolves the surface
of the mineral resulting in a “wet”, or greasy feel on your fingers.
3. Taste
A few minerals readily dissolve in the range of temperatures and
pressures found on the Earth’s surface. The result is often a distinctive
taste to the surface of the mineral. Halite, also known as salt (sample #73),
is a common mineral with a characteristic taste. Most geologist frown at
the idea of mineral taste tests.
IMPORTANT NOTE
We recommend that you DO NOT taste any of the minerals in the lab.
It is, however, usually safe to taste minerals in the field after they have
been rinsed with water.
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GEOL 1107, Lab #1
4. Thermal Effects
Some minerals have distinctive reactions to heat. The mineral vermiculite
is a sheet-like clay mineral that expands greatly when exposed to a flame.
Biotite, which has the same sheet-like structure as vermiculite, is not
affected by heat.
The explanation for the different thermal properties of these two minerals
is found in their chemical formulas.
The chemical formula for biotite is K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2
The chemical formula for vermiculite is (Mg,Fe,Al)3(Al,Si)4O10(OH)2 . 4H2O
There’s a lot of different atoms in each of these minerals, they both have a
complex ratio of atoms. However, it’s what’s at the end of the chemical
formula for vermiculite that causes it to expand when heated. At the end
is 4H2O which means that water (H2O) is an integral part of vermiculite.
When the mineral is heated the water boils off causing the mineral to
expand. Biotite on the other hand does not contain water and is not
affected in the same way when heated.
EXERCISE 1 - 8: THERMAL EFFECTS
Hold a sample each of vermiculite and biotite over the flame of a match and
note what happens. Examine the chemical formula (given below) for each of
these minerals.
Which mineral reacts to the heat and which does not?
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GEOL 1107, Lab #1
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GEOL 1107, Lab #2
LABORATORY 2
CLEAVAGE AND FRACTURE
CLEAVAGE
Cleavage, as it pertains to minerals, is a particular way that a mineral breaks
or cleaves. Depending upon the internal atomic structure of a mineral it may
break or cleave along a plane or planes of weakness. The result is that, as a
mineral is broken, relatively smooth flat surfaces appear. For example, a raw
diamond (Figure 2 - 1) may have a distinctive, prismatic shape or be an
irregular mass with no smooth flat surfaces. However, when a diamond is
cut or cleaved it will often have a very distinct, aesthetically pleasing design
often seen in jewelry.
Figure 2 - 1. Raw diamonds.
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GEOL 1107, Lab #2
IMPORTANT NOTE
Cleavage is defined by the smooth flat surfaces that appear when a
mineral breaks. Do not confuse this with crystal habit, which also is
defined by smooth flat surfaces, but these are formed when a mineral
crystal grows. However, the cleavage of a mineral is often the same
or very similar to its crystal habit.
The cleavage of a mineral is defined by the number of flat surfaces that
appear when a mineral breaks, the quality of the cleavage surfaces and the
angle between them. The quality of cleavage refers to how smooth and
continuous surfaces are and therefore how easily they are observed. Smooth,
continuous surfaces describe good or even perfect cleavage. A rough,
discontinuous surface is poor cleavage. The mineral biotite, for example,
when broken separates into thin, flat plates, the surface of which is a cleavage
surface (Figure 2 - 2 A). While there are two surfaces, the top and bottom of
the mineral that are parallel, biotite has only one type of surface. Biotite
therefore has one cleavage because there is only one type of surface. Also
because biotite only has one type of cleavage surface there is no way to
measure an angle between cleavage surfaces.
The common minerals pyroxene and hornblende both have two poor to good
cleavage surfaces (Figure 2 - 2 B). This means that when they break it is
possible to see two types of smooth, flat surface. Key to distinguishing
pyroxene from hornblende is the cleavage angle, the angle between adjacent
cleavage surfaces. The cleavage angle for pyroxene is 93o or 87o, which for all
intents and purposes is 90o or a right angle. The cleavage angle for
hornblende is 56o or 124o. Sometimes it’s described more simply as ‘not 90o’.
Cleavage becomes increasingly more complex as the number of cleavage
surfaces increases. Cleavage in three directions is characteristic of the
common minerals galena and calcite (Figure 2 - 2 C). However, galena and
calcite have different cleavage angles. The angle between adjacent cleavage
surfaces in galena is 90o which imparts a cubic shape to a cleaved sample.
The cleavage for galena is sometimes simplified by calling it cubic. The
cleavage angle for calcite is not 90o, or more accurately measured as 60o or
120o. The cleavage of calcite is sometimes referred to a rhombic or
rhombohedral.
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GEOL 1107, Lab #2
B. Pyroxene and hornblende have good cleavage in two directions.
Pyroxene at 90°. Hornblende at 60°/120°.
Figure 2 - 2. Types of cleavage: A Single, platey biotite sample showing perfect or good cleavage in
1 direction.; B Pyroxene and hornblende cleavage, in 2 directions at approximately 90o and at
approximately 60o or 120o. C Galena and calcite cleavage in 3 directions at 90o and not at 90o. D
Fluorite and diamond cleavage in 4 directions.
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GEOL 1107, Lab #2
The Quality of Cleavage - Perfect, Good and Poor.
Minerals with cleavage may display a pattern of flat, shiny surfaces with
varying degrees of quality. Perfect or good cleavage, for example, results when
the mineral breaks along a more continuous, smooth plane or surface (Figure 2 3 A). What is often more common is “poor” cleavage whereby the mineral
breaks or cleaves along a series of small planes or surfaces that are not
continuous and therefore do not form one large smooth surface (Figure 2 - 3 B).
When cleavage is poor it is still possible to observe the number and orientation
of cleavage surfaces, it just takes closer examination. You may have to use a
microscope or enlarge an image. Another way is to move the mineral at
different angles to a light source to see how all these small cleavage surfaces
reflect light the same way.
A
B
Figure 2 - 3. A Good cleavage along a smooth, continuous surface. B Poor cleavage
along a more irregular, discontinuous surface formed by a series of smaller cleavage
surfaces.
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GEOL 1107, Lab #2
FRACTURE
Fracture is different from cleavage. Fracture is when a mineral breaks and
there is no flat surface or cleavage plane. A mineral with cleavage may break
but perhaps not along one of its cleavage planes. In addition, not all minerals
have cleavage. In this case when the mineral breaks it “fractures” along
irregular surfaces which are not flat and there is no consistent angle between
adjacent surfaces. The mineral quartz for example has no cleavage.
Therefore, when quartz breaks it does not cleave or separate along smooth,
continuous surfaces. Instead it fractures along rough irregular surfaces.
There are two common forms of fracture: 1. uneven; and 2. conchoidal.
1. Uneven fracture refers to any mineral that has no cleavage and
instead breaks along rough, irregular surfaces. The mineral quartz
(Figure 2 - 4) has an uneven fracture.
Scale is in centimeters
Figure 2 - 4. The uneven fracture of quartz.
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GEOL 1107, Lab #2
2. Conchoidal fracture refers to a smoothly curving fracture. Smooth
continuous convex or concave surfaces appear when minerals such as
microcrystalline quartz break (Figure 2 - 5). When more than one
conchoidal surface is present on a single mineral they frequently
intersect at sharp angles thereby imparting a “knife-edge”
appearance to the mineral.
Scale is in centimeters
Figure 2 - 5. The conchoidal fracture of microcrystalline quartz
with its smooth curved surfaces.
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GEOL 1107, Lab #2
Here is some useful information that can help with cleavage as a property for
mineral identification.
1. With a hand sample it is possible to observe cleavage when a mineral is
moved or turned around until a flash of reflected light is seen from a
particular location on the sample. This may be a reflection from a
cleavage surface. The reflection may be from a single surface or it may
come from many small parallel surfaces on the sample. Cleavage may
also be seen because of thin parallel cracks on a surface of the mineral.
Each set of parallel cracks and the surfaces they are on represent a
cleavage. To look for additional cleavage, turn the sample again and look
for more reflections. Estimate the angle between cleavages by rotating
back and forth between reflections.
2. Cleavage angles need not be measured precisely. Estimate them carefully
with the naked eye and determine whether the angle is larger, smaller or
equal to 900.
3. Sometimes the use of a geometric term that describes the shape of a
minerals cleavage is the best method of giving both number and angles of
cleavage. For example, cubic cleavage implies 3 types of cleavage
surfaces, in three different directions, all at 900 to one another (Figure 2 - 2
C). Rhombic cleavage has also has 3 types of flat surface, in three different
directions, but the angle is not 900 (Figure 2 - 2 C), and octahedral implies
4 cleavages resulting in a regular 8 sided form with more than one angle
(Figure 2 - 2 D). Either way can be used to describe the cleavage of a
mineral, listing the number, orientation and angles between surfaces, or
by using a geometric term.
EXERCISE 2 - 1: DETERMINING CLEAVAGE
The property of cleavage is essential for recognizing and distinguishing
between many different kinds of minerals. Table 2 - 1 is a list of common
minerals, all of which may or may not have some sort of cleavage. Determine
the cleavage for these 9 common minerals and record your observations in
table 2 - 1. Be sure to check and discuss your results with your instructor.
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GEOL 1107, Lab #2
IMPORTANT NOTE
Please do not break the minerals as good samples are in limited supply.
Crushed samples of the minerals biotite, pyrite, galena and halite have
been provided for you in the lab display. The other minerals are found in
the box of common minerals. Use the samples from more than one
common mineral box so you can observe a number of different examples.
MINERAL
NO. OF
CLEAVAGES
ANGLE(S)
QUALITY
Galena #60
Perfect
Quartz #131
?
Calcite # 26a
Good
Biotite # 24
Perfect
Feldspar # 101
Poor
Hornblende # 78
Poor
Halite # 73
Good
Fluorite # 57
Good/Perfect
Pyrite # 124
?
Table 7. Common minerals and their cleavage properties.
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GEOL 1107, Lab #3
LABORATORY 3
FELDSPAR MINERALS
Feldspars are one of the most abundant groups of minerals in the Earth’s
crust and hence are geologically very important.
Two distinctly different groups of feldspar minerals are distinguishable using
readily observed properties. They are:
1) the plagioclase feldspars;
2) and the potassium or K-feldspars.
Each group includes a number of different minerals. This lab will focus on
these two types which are easily recognized in a hand sample. In many other
cases it takes special sample preparation and laboratory analysis to properly
identify a feldspar mineral.
PLAGIOCLASE FELDPSARS
Plagioclase feldspars are a group of silicate minerals whose chemical
composition varies with changes in the amount of calcium (Ca) and sodium
(Na) in the atomic structure. Plagioclase feldspars are a continuous solid
solution series meaning that chemical composition ranges from a pure Cabearing feldspar (Anorthite) to a pure Na-bearing feldspar (Albite) with all
sorts of mixtures of Ca and Na in between.
POTASSIUM FELDSPARS
Potassium or K-feldspars are, like the plagioclase feldspars, a silicate mineral,
with K as the dominant cation present in the atomic structure. Common
varieties of K-feldspar include microcline and orthoclase.
FELDSPAR IDENTIFICATION
Striations vs Perthitic Texture
The two common types of feldspar, plagioclase and potassium, while similar
in many ways, can be readily distinguished by a few diagnostic features.
Primary among them are striations on the plagioclase feldspar and a perthitic
texture which is common on K-feldspars (Figures 3 - 1 and 3 - 2).
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GEOL 1107, Lab #3
Striations
Blue-green
iridescence
Figure 3 - 1. Plagioclase feldspar texture. Striations on cleavage surfaces of a
plagioclase feldspar mineral. This also shows the iridescence characteristic of this
mineral.
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GEOL 1107, Lab #3
Perthitic Texture
Scale is in centimeters
Figure 3 - 2. Potassium feldspar texture. A perthitic texture on the cleavage surface
of this mineral.
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GEOL 1107, Lab #3
The surface of a plagioclase feldspar mineral is characterized by striations
(Figure 3 - 1). These are very thin, only a fraction of a millimeter wide, very
straight, parallel, continuous lines that appear on cleavage faces on the
mineral.
Striations result from crystal twinning that occurs during mineral growth. A
twinned plagioclase crystal is actually a composite crystal which means it is
composed of more than one crystal face that have different but related
orientations with respect to each other. A twinned plagioclase crystal is
composed of numerous, thin, parallel crystal faces arranged in alternating
fashion such that the mineral appears to have a lined or striated surface. A
twinned relationship results in a slight deflection of the cleavage surface, and
this shows up as a series of parallel lines referred to as striations.
Potassium or K-feldspars do not have striations. Instead, they may have what
is called a perthitic texture (Figure 3 - 2). The perthitic texture of K-feldspar is
characterized by numerous irregular or wavy, short discontinuous lines, of
variable thickness, usually not more than 1 mm wide. They appear on the
surface of the mineral and are not parallel or sub-parallel to one another.
They are significantly thicker, and therefore much easier to see, than the very
thin, straight, parallel striations on a plagioclase feldspar mineral.
The perthitic texture of a K-feldspar mineral is the result of small inclusions
of Na-bearing feldspar. Potassium feldspar forms at high temperatures, and
typically contains some amount of dissolved Na-bearing feldspar. As the
melted mineral cools and starts to solidify, the Na-bearing feldspar portion
tends to exsolve or “unmix” from the K-feldspar. The exsolved Na-bearing
feldspar takes the form of thin, sub-parallel inclusions within the rest of the
K-feldspar mineral, thereby giving it that distinctive “perthitic” or “lined”
appearance.
Plagioclase feldspar is also easy to identify because of the blue-green
iridescence it sometimes has when light reflects off its surface (Figure 3 - 1).
Iridescence is a property unique to a few minerals, but plagioclase is amongst
the most common. It appears as if this mineral is changing colour from green
to blue to white, as light reflects at different angles off its surface. It can be a
key to proper identification of this feldspar mineral.
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GEOL 1107, Lab #3
Table 3 - 1 summarizes the main points associated with feldspar mineral
textures and identification.
PLAGIOCLASE FELDPSPAR
POTASSIUM or K-FELDSPAR
Striations are common to rare
depending on quality of mineral
Perthitic texture is common,
Perthitic texture characterized by thin,
lens-shaped inclusions. These are
thin, approx. 1 mm, discontinuous,
sub-parallel, linear inclusions.
Thin, << 1 mm, straight, parallel
striations
Striations parallel to the edge of
cleavage surface
Striations appear on only one cleavage
surface, only rarely on 2 surfaces
Lines cut obliquely across the edge of
a cleavage surface at approx. a 600 - 900
angle
May be seen on both cleavage surfaces
Colour is typically white, but may vary
Blue – green iridescence on cleavage
surfaces
Colour is commonly pink, orange or
white, but may vary
Lines may or may not be visible
depending upon the colour of the
mineral
Table 3 - 1. Main points about texture, cleavage and colour used to identify
plagioclase and potassium or K-feldspar minerals.
Here’s something to think about when using striated or perthitic textures to
identify a feldspar mineral.
First locate the cleavage planes, both plagioclase and K-feldspar minerals
have two poor to good cleavages at approximately 90o. Look for striations or
a perthitic texture. Similarly, you may first want to locate the texture of the
feldspar, either striated or perthitic, and use this feature to help identify
cleavage surfaces.
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GEOL 1107, Lab #3
EXERCISE 3 - 1:
PLAGIOCLASE vs K-FELDSPAR MINERALS
Examine first the samples in the lab at the “feldspar” display. Be sure you
observe each of the following:
1. cleavage surfaces, cleavage edge
2. striations on plagioclases
3. perthitic structure in K feldspars
4. iridescent luster in plagioclase
5. colour variations
Also examine the minerals in the box of common minerals.
List the sample number and texture of all the mineral samples from the
“feldspar” display that are plagioclase feldspars and the sample number and
texture of those that are potassium or K-feldspars.
Sample No. Texture (Perthitic or Striated?)
Feldspar (Plagioclase or “K”?)
IMPORTANT NOTE
All the feldspar minerals are extremely important for future labs.
Be sure you learn to identify them.
6
GEOL 1107, Lab #4
LABORATORY 4
MINERAL IDENTIFICATION
Based on the physical properties of minerals introduced in labs 1, 2, and 3 it’s
now time to start identifying the common minerals included in this course.
To identify a mineral, you need to determine what it is and why you think
so.This means you give the mineral a name and list the properties you use to
identify it.
For example, take the mineral in figure 4 - 1. We don’t know what this
mineral is but right away we can list some of its properties.
Figure 4 - 1. Unknown mineral
1. Let’s start with luster.
The mineral has a non-metallic luster, meaning it does not look like a piece of
metal. More specifically, it has a bright glassy type of non-metallic luster.
1
GEOL 1107, Lab #4
2. Then there’s colour. The mineral is primarily white with clear or
transparent parts around the edges.
3. What about fracture or cleavage?
There are no smooth, flat surfaces that may be cleavage. The surface of this
mineral is instead irregular with an uneven fracture. It’s safe to assume then
that it has no cleavage.
4. What about hardness?
Using the tools provided in the lab, we’ve determined a hardness of
approximately 7.
5. What about streak?
Again, using the tools provided in the lab the streak is white.
Now we have five properties, more than enough to identify the mineral.
There are other properties we can also consider. The mineral in figure 4 - 1 is
non-magnetic and does not react with hydrochloric acid (HCl). These are
helpful things to know but they don’t tell us what properties the mineral has,
instead they tell us what properties the mineral does not have, and lots of
minerals are non-magnetic and don’t react with HCl. Do not use properties
the mineral does not have to identify it.
IMPORTANT
Don’t describe what the mineral does not have for properties. Describe the
properties that the mineral does have.
So, for the mineral in figure 4 - 1 we have five properties. Throughout this
course you will always be asked to provide three to five properties that you
see in a mineral (i.e. colour), or determine by examining it closely (i.e.
reaction with HCl) that are used to identify it.
The next step is to find the name of the mineral. For this you go to table 4 - 1
or table 4 - 2 in this lab.
2
GEOL 1107, Lab #4
Table 4 - 1, beginning on page 12, has all the metallic minerals you will need
to know for this course. The minerals in table 4 - 1 are listed from softest to
hardest. The first mineral is molybdenite with a hardness of 1 to 1.5. The last
mineral is table 4 - 1 is pyrite which has a hardness of 6 to 6.5.
Table 4 - 2, a much longer table beginning on page 16, includes all the nonmetallic minerals you need to know. These minerals are also listed from
softest to hardest beginning with talc, which has a hardness of 1, and ending
with diamond, with a hardness of 10.
The mineral in figure 4 - 1 is non-metallic so it’s found in table 4 - 2. It has a
hardness of 7. The first mineral in table 4 - 2 with a hardness that includes 7
is olivine and the last mineral with a hardness of 7 is quartz. In between are
four more minerals. So, we’ve narrowed down the number of possible
answers to six minerals.
Now let’s look at colour. The mineral in figure 4 - 1 is mostly white with
some clear or colourless parts. Going back to table 4 - 2, what mineral has a
hardness of 7 and is white or colourless? The only two minerals that fit this
description are kyanite and quartz. Kyanite, however, often has a blue colour,
which is nowhere to be seen in figure 4 - 1. So, by the process of elimination
we’ve determined that the best answer when identifying the mineral in figure
4 - 1 is quartz. When we compare the image in figure 4 - 1 to the images of
quartz in table 4 - 2 and other images like figure 1 - 4 A in lab 1, it’s pretty
clear that the mineral is quartz. So, that’s how mineral identification can
work.
Based on a description of the mineral’s properties and a comparison with
mineral descriptions and images in labs 1, 2 and 3 a name is selected, and
themineral identified.
The important thing to remember is that this is a science course, and we want
you to think like a scientist, which means using the scientific method. First you
ask a question, like what is the name of the mineral I’m looking at? Then you
gather evidence, like mineral properties, that will help answer the question.
Based on this evidence you come up with an answer. Then you test you answer
by looking at the information in labs 1, 2, 3 and 4. If the answer makes sense
you’re done, and you move on to the next mineral. If your answer does not
make sense then you need to confirm the properties you have, get more if need
3
GEOL 1107, Lab #4
be and then rethink your answer. At the end of the day, you do the best you
have with the information at hand. Geologists do it all the time. Remember the
really important point first stated at the beginning of lab 1.
THIS IS REALLY IMPORTANT
READ THIS AND SAVE YOURSELF SOME TROUBLE LATER.
A really important thing to remember about all minerals is that they vary.
What this means is that some minerals will have a very specific property that
make them easy to identify, such as the emerald green colour of malachite.
Other minerals will have a number of different properties that also make it
easy to identify them. The mineral plagioclase feldspar has a white colour, thin
straight lines or striations on its cleavage surface and a blue - green iridescence.
Many minerals have a property or properties that are very useful for
identifying them, but because they vary you may not see them in all samples of
the same mineral. So how then do you identify a mineral? The answer is
simple. Identification is based not on one but as many properties as possible.
Also make sure you look at more than one sample of the same mineral so you
can see as many properties as possible and how they might look different. List
all the properties you see and make a decision, chances are you’ll be right. At
the end of the day, the name you give to a mineral is based on the properties
you see and the sum total of what they tell you.
A mineral identification flow chart called “How to identify a mineral” is
another method that you may find useful and are welcome to use. It asks a
series of questions that have either a ‘YES’ or ‘NO’ answer that then directs
you to the next question and eventually a name for the mineral. It works well
for most of the common minerals in this course, but not all. For some
minerals it finishes with a short list of possible names from which a final
selection is made. Use this flow chart if you want. Use any method you want
to identify a mineral. We’re less concerned with how you get an answer,
that’s up to you, and more concerned with what the answer is. All you need
do is name the mineral and list the properties used to identify it. We ask that
you include from 3 to 5 properties that you can see for yourself (i.e. colour) or
are given to you (i.e. streak, hardness).
4
GEOL 1107, Lab #4
5
GEOL 1107, Lab #4
6
GEOL 1107, Lab #4
7
GEOL 1107, Lab #4
8
GEOL 1107, Lab #4
EXERCISE 4 - 1: MINERAL IDENTIFICATION
Based on the physical properties of minerals introduced in lab 1, crystal habit
in lab 2, cleavage in lab 3 and feldspar properties in lab 4, identify the
different minerals in the common and economic mineral boxes.
For each mineral list three to five properties that you either see in the mineral.
Use mineral identification table 4 - 1, Metallic Minerals, beginning on page 12,
and table 4 - 2, Economic Minerals, beginning on page 16. Use the flow chart,
entitled “How to identify a mineral” starting on page 5 as one method to aid
with identification.
As you identify a mineral fill in the charts on pages 10 and 11. In the box
with the appropriate sample number include a list of the mineral properties
and the name of the mineral.
This exercise is open book so use all your labs. Get to know your labs and
where to find information in them. All future quizzes and lab tests will be
open book. After you’ve had some practice you may want to time yourself to
see how long it takes. The only condition is all quizzes and tests will have a
limited time to complete them.
Also included in lab 4 is a list of the common mineral and their economic use
(Table 4 - 3). This will help you better appreciate some of the importance of
rocks and minerals and how they affect our everyday life.
9
GEOL 1107, Lab #4
Common mineral identification chart
10
GEOL 1107, Lab #4
Economic mineral identification chart
11
Table 4 - 1. Metallic minerals with hardness less than 2.5 (can be scratched with a fingernail)
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Bluish “steel”
gray metallic
color to silvery
gray
Black to blue
gray, grayish
green when
marked on paper
1 – 1.5
1 perfect cleavage.
Not commonly
observed. Usually
appears with irregular
fracture.
High density (4.7g/cm3).
Greasy feel. Metallic
luster.
Molybdenite
MoS2
Sulfides
Silvery gray to
dull black
Gray to black
1-2
1 perfect cleavage.
Not commonly
observed. Usually
appears with irregular
fracture.
Low density (2.2 g/cm3).
Greasy feel. Usually
bright metallic luster,
sometimes sub-metallic
to dull luster.
Graphite
C
Native
Elements
Bright silvery
metallic to dull
gray.
Dark gray to black
2.5 – 2.8
3 perfect @ 90o
imparts cubic shape.
Very high density
(7.5 g/cm3). Commonly
forms perfectly cubic
crystals that mimic
cleavage. Metallic
luster.
Galena
PbS
Sulfides
12
Table 4 - 1. Metallic minerals with hardness between 2.5 and 5.5 (harder than a fingernail, can be scratched by glass)
COMMON
MINERAL
ASSOC.
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
Yellow. May
vary depending
upon the
presence of
impurities.
Yellow
2.5 - 3
No cleavage. Hackly
fracture.
High density (19.3
g/cm3). Gold’s higher
density distinguishes it
from pyrite and some
mica flakes which are
similar in appearance.
Occurs in irregular,
compact masses. Very
ductile and malleable.
Metallic luster.
Native Gold
Au
Native
Metals
Copper to red
metallic.
Tarnishes dull.
Light green to
turquoise when
exposed to air.
Copper to red
metallic.
2.5 - 3
No cleavage. Hackly
fracture.
High density
(8.9 g/cm3). Occurs in
irregular, compact
masses. Sometimes
dendritic. Very ductile
and malleable. Metallic
luster.
Native
Copper
Cu
Native
Metals
Yellow metallic
“bronzy” when
newly exposed.
Tarnishes to
iridescent purple
and blue.
Gray to black.
3
Poor cleavage not
usually observed.
Commonly has
irregular fracture
sometimes poorly
conchoidal.
High density (5.7 g/cm3).
May appear as a dense,
granular mass. A
metallic ore, sometimes
called “peacock” ore.
Metallic luster.
Bornite
Cu5FeS2
Sulfides
13
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
CuFeS2
Sulfides
Pyrrhotite
Fe(1-x)S
(variable but close
to FeS)
Sulfides
Sphalerite
(Zn,Fe)S
Sulfides
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
Dark or brassy
yellow, often
with iridescent
film when
tarnished.
Greenish black
3.5 - 4
No cleavage.
Uneven to
conchoidal fracture.
Generally in compact,
massive or microgranular
masses. Metallic luster.
Chalcopyrite
Yellow bronze to
bronze and dull
gray brown.
Gray black
3.5 - 4.5
No cleavage.
Uneven fracture.
Common as massive,
granular aggregates.
Commonly magnetic, a
property that helps
distinguish it from nonmagnetic pyrite.
Metallic luster.
Commonly black
to brown. Color
varies depending
upon changes in
composition and
the presence of
impurities
Brown to
yellowish brown
to reddish.
3.5 - 4
6 good cleavages that
parallel a complex
“12-sided” crystal
structure. All 6
cleavage surfaces are
rarely seen in one
sample. Cleavage
and crystal habit
make for a very
complex appearance
which in itself may
be diagnostic.
Sub-metallic luster,
sometimes metallic in
appearance.
14
Table 4 - 1. Metallic minerals with hardness between 5.5 and 6.5 (harder than a steel knife, softer than a streak plate)
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Dull red to
reddish brown or
bright silvery to
bluish gray
metallic.
Red to reddish
brown
5.5 – 6.5
No cleavage.
Uneven fracture but
not commonly
observed.
Massive, compact,
amorphous, granular
masses. Frequently
colloform or
concretionary in texture.
Bright, metallic or dull
earthy luster depending
upon variety.
Hematite
Fe2O3
Oxides
Black
5.5 – 6.5
No cleavage.
Uneven fracture.
Compact and granular
masses. High density
(5.2 g/cm3). Occurs
Strongly Magnetic.
Submetallic to vitreous
luster.
.
Magnetite
Fe3O4
Oxides
Greenish to
brownish black
6 – 6.5
No cleavage.
Fracture commonly
uneven, may be
conchoidal.
Occurs as compact,
granular aggregates.
Frequently has “perfect”
4-sided cubic and more
“irregular” 8-sided
octahedral crystals with
striated surfaces. High
density (5.0 g/cm3).
Very fragile. Produces a
spark if struck by a
hammer. Metallic luster.
Pyrite
FeS2
Sulfides
Black
Yellow to brassy
yellow.
15
Table 4 - 2. Non-metallic minerals with hardness less than 2.5 (can be scratched with a fingernail)
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Commonly
white, maybe
greenish white,
gray or brownish.
White
1
1 perfect cleavage
rarely seen.
Very soft, compact,
massive, easily carved
“steatite” structure.
Greasy feel when
massive. Never occurs
in distinct crystals.
Usually an earthy luster,
sometimes pearly.
Talc
Mg3Si4O10
(OH)2
Sheet
Silicates
Yellow to black
(Variable)
Yellow brown
1-5
(Variable)
Depending upon
variety of mineral,
cleavage not present
or unobserved.
Fracture uneven.
Usually too small to
see.
Limonite is a general
term used to describe a
number of amorphous,
difficult to define iron
oxide and hydroxide
minerals, such as
goethite. These may
occur as colloform or
non-crystalline earthy
masses, crusts or stains.
Common dull to earthy
luster. Usually a
secondary mineral
formed through the
weathering and oxidation
of pyrite. Dull, earth
luster.
Limonite
FeOOH
Goethite
Oxides/
Hyroxides
16
FeO(OH)
Lepidocrocite
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
White to
yellowish brown
to brown.
White
2 – 2.5
1 perfect cleavage
rarely seen.
Usually a soft, compact,
massive or very finegrained aggregate. Has
the odor of freshly turned
soil when newly
exposed. Dull, earthy
luster.
Kaolinite
Al2Si2O15
(OH)4
Sheet
Silicates
Golden yellow to
brown.
Pale yellow
2-3
1 perfect cleavage.
Low density (2.4 g/cm3).
Irregular masses of platy
crystals that appear as
scaly aggregates. When
heat to >300oC crystals
lose water and expand 18
to 25 times original
volume. Dull to vitreous
luster.
Vermiculite
(Mg,Fe,Al)3
(Al,Si)
4O10
.
(OH)2 4H2O
Sheet
Silicates
White, gray,
yellowish, pink
to brown.
White or pale
colored.
2
1 good, 2 poor
cleavages, difficult to
observe.
Maybe granular and
compact waxy-looking
masses or fibrous
aggregates of elongated
crystals. Vitreous luster.
Gypsum
CaSO4
.2H O
2
Sulfates
17
COLOR
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Perfect cleavage
rarely seen.
High density (8.1 g/cm3),
although difficult to
determine as it rarely
occurs in discrete
masses. Microcrystalline
or earthy masses, films
or disseminated grains.
Dull, earthy luster.
Cinnabar
HgS
Sulfides
2 – 2.5
1 perfect cleavage.
Dark, compacted masses
of platy crystals.
Vitreous luster.
Chlorite
2 – 2.5
1 perfect cleavage.
Foliated or layered,
scaley masses of very
thin, platy crystals.
Individual crystals are
flexible, elastic and may
be very large (>10 cm).
Crystals are transparent
or translucent with pearly
to glassy luster.
Muscovite
STREAK
HARDNESS
Bright red to
scarlet.
Scarlet to orange red
2 – 2.5
Greenish black
Greenish black
Colorless to
silvery-white,
white or yellow.
Colorless to white
CLEAVAGE/
FRACTURE
18
(Fe,Mg,Mn,Ni,Al)
6AlSi3O10(OH)8
KAl2(AlSi3)O10
(OH)2
Sheet
Silicates
Sheet
Silicates
Table 4 - 2. Non-metallic minerals with hardness between 2.5 and 5.5 (harder than a fingernail, can be scratched by glass)
COMMON
MINERAL
ASSOC.
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
Black, brown or
dark green.
Black, brown or
dark green.
2.5 - 3
1 perfect cleavage.
Foliated or layered, scaley
masses or aggregates of
very thin, platy crystals.
Individual crystals are
flexible, elastic, may be
large. Transparent or
translucent. Vitreous
luster.
Biotite
K(Mg,Fe)3
(Al,Fe)Si3O10
(OH,F)2
Sheet
Silicates
Yellow to dark
green or white
Yellow to dark
green or white
2.5 - 4
1 good cleavage
(Antigorite), usually
difficult to observe,
or no cleavage
(Asbestos)
Serpentine
(Mg,Fe)3
Si2O5(OH)4
Sheet
Silicates
Frequently
colorless, but
may be white,
pale yellow, red,
brown or black
depending upon
impurities.
White. May vary
depending upon
color.
2.5
3 perfect cleavages
@ 90o.
Halite
NaCl
Halides
19
Depending upon the
variety of serpentine it
may be a hard, dense mass
of laminated, irregular
crystals (Antigorite) or a
more lose aggregate of
elastic fibrous crystals
easily separated
(Asbestos). Greasy to
pearly luster.
Low density (2.1 g/cm3).
May appear as large
individual crystals, dense
clusters of well formed
crystals or
microcrystalline crusts or
coatings. Vitreous luster.
Dissolves readily in water.
Distinctive “salty” taste.
COLOR
STREAK
HARDNESS
White or
yellowish to red,
reddish brown to
brown.
White or
yellowish to red,
reddish brown to
brown. May vary
depending upon
color.
1-3
Commonly
white, less
commonly
colorless, or pale
yellow, red or
green.
White
2.5 – 3.5
Colorless, white,
pink, green,
yellow or black.
White
3
CLEAVAGE/
FRACTURE
COMMON
MINERAL
ASSOC.
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
Bauxite refers to a group
of related minerals
including gibbsite,
boehmite, and diaspore.
Found in earthy, massive
or pisolitic masses.
Earthy luster.
Bauxite
Al(OH)3
Gibbsite,
3 good cleavages
forming a prismatic
shape.
High density (4.5 g/cm3),
unusual for a non-metallic
mineral. Typically
compact, granular masses.
Vitreous luster.
Barite
BaSO4
Sulphates
3 perfect cleavages
@ approximately
60o.
Occurs as distinct
rhombohedral or prismatic
crystals, inter-growths of
crystals, or compact,
dense, microcrystalline
masses that occur as
encrustations or void
fillings. Vitreous to
iridescent, pearly luster.
Soluble in cold, dilute
(10%) hydrochloric acid
where it effervesces. A
common biogenic
material in many marine
invertebrates (bivalves,
coral).
Calcite
CaCO3
Carbonates
No cleavage.
Uneven fracture.
20
Oxides/
Hyroxides
AlO(OH)
Boehmite and
Diaspore
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Colorless, white,
pink, green, or
yellow.
White
3.5 - 4
3 perfect cleavages
@ approximately
60o.
Occurs as distinct
rhombohedral crystals,
aggregates of crystals, or
compact, dense, coarse
crystalline or
microcrystalline masses.
Vitreous to iridescent,
pearly luster. Soluble in
cold, dilute (10%)
hydrochloric acid only
when fragmented to a fine
powder where it
effervesces
Dolomite
CaMg(CO3)2
Carbonates
Bright green or
emerald green.
Light green
3.5 - 4
Good cleavage,
rarely seen.
Malachite
Cu2(CO3)(OH)2
Carbonates
Bright blue
Pale blue
3.4 - 4
Moderately good
cleavage
Azurite
Cu3(CO3)2
(OH)2
Carbonates
Common as a green film
or stain on other copperrich minerals. May occur
as thinly laminated,
colloform, reniform
(“kidney-shaped”)
masses. Vitreous or silky
luster.
Commonly occurs as a
blue earthy film or stain
on other copper-rich
minerals. May occur as
thinly laminated,
concretionary or granular
mass. May also occur as
groups of radiating,
elongated or tabular
crystals. Vitreous luster.
21
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Colorless to
yellow, green,
blue, pink, purple
or black.
Colorless or white
to variable.
4
4 perfect cleavages
forming octahedral
(8 – sided double
pyramid shape)
structure.
Cubic crystals. May also
occur as compacted,
banded and concretionary
masses. Crystals intergrowths are common.
Vitreous luster.
Fluorite
CaF2
Halides
Colorless to
yellow, green,
brown, less
commonly blue
or red.
Always white
5
1 poor cleavage.
The apatite group includes
a number of different
minerals including
fluorapatite. Hexagonal
(6-sided) crystals may be
elongated or stubby. Also
appears as granular or
colloform masses.
Vitreous to sub-resinous
luster.
Apatite
Ca5F(PO4)3
Phosphates
Black to dark
green
Black
5-6
2 good cleavages @
56o and 124o.
Part of the amphibolite
group of minerals that
also includes Tremolite
and Actinolite. Generally
short, stubby crystals.
Frequently elongated,
acicular to fibrous
crystals. Vitreous luster.
Hornblende
(Ca,Na)2-3
(Mg,Fe+2,Fe+3,
Al)5(Al,Si)8O22
(OH)2
Double
Chain
Silicates
22
Table 4 - 2. Non-metallic minerals with hardness between 5.5 and 6.5 (harder than a steel knife, softer than a streak plate)
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Dull red to
reddish brown or
bright silvery to
bluish gray
metallic.
Red to reddish
brown
5.5 – 6.5
No cleavage.
Uneven fracture but
not commonly
observed.
Massive, compact,
amorphous, granular
masses. Frequently
colloform or
concretionary in texture.
Bright, metallic or dull
earthy luster depending
upon variety.
Hematite
Fe2O3
Oxides
Commonly
black, or dark
greenish black to
brown or black.
Sometimes
yellowish green
depending upon
variety.
Commonly black
or gray-green.
5-6
2 good to perfect
cleavages @87o.
Sometimes a poorly
defined conchoidal or
splintery fracture.
The pyroxene group
includes a number of
minerals. Generally
prismatic crystals often
stubby, square or
octagonal in cross-section.
Sometimes massive or
granular aggregates
variety. Vitreous luster.
Pyroxene
Mg2Si2O6
Enstatite,
Single
Chain
Silicates
23
(Ca,Na)
(Mg,Fe,Al,Ti)
(Si,Al)2O6
Augite,
CaMgSi2O6
Diopside
COLOR
STREAK
HARDNESS
Colorless, white,
yellowish, green,
less commonly
pink or reddish.
White to colorless
6
Typically
colorless, pink or
white.
Sometimes
green, yellow,
blue or gray.
White
6
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
2 good cleavages
@87o.
Plagioclase feldspars
include a group of
minerals that show
different physical
characteristics depending
upon the ration of Ca and
Na cations. Usually in
compact, granular masses.
Glassy to pearly luster.
Plagioclase feldspars may
have well defined, very
thin, straight, parallel
“hair-like” striations.
Sometimes strongly
iridescent (when it is the
Labrador variety).
Plagioclase
Feldspar
(Ca,Na)(Al(Si,Al)
Si2O8
Framework
Silicates
2 good cleavages
@90o.
Potassium (K) or alkali
feldspars include a group
of similar minerals with
only slight variations in
physical appearance.
Compact, granular masses
are common. Large to
very large prismatic,
columnar or tabular
crystals are common.
Glassy to pearly luster.
K-feldspars frequently
have a well defined,
“perthitic” texture.
Potassium
Feldspar
KAlSi3O8
Framework
Silicates
CLEAVAGE/
FRACTURE
24
Table 4 - 2. Non-metallic minerals with hardness greater than 6.5 (harder than a streak plate)
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Olive green,
yellowish green
to yellow.
Green
6.7 - 7
1 poor cleavage.
Frequently has
conchoidal fracture.
Olivine defines a small
group of minerals.
Typically small prismatic
crystals arranged in a
dense, compacted granular
mass or aggregate.
Vitreous luster.
Olivine
Mg2SiO4
Forsterite,
Island
Silicates
Fe2SiO4
Fayalite
Green to
yellowish green.
Green to
yellowish green.
6-7
1 good cleavage.
Prismatic, elongated
columnar crystals
arranged in granular
masses or aggregate of
radiating “fibrous-like”
crystals. Vitreous luster.
Epidote
Ca2(Al,Fe)3Si3O12
(OH)
Island
Silicates
Light blue color
that varies in
intensity within
individual
crystals. Less
commonly white
or gray.
White
6–7
(across
cleavage
planes),
1 perfect cleavage
Elongated, tabular
crystals. Larger,
individual crystals and
clusters or aggregates of
radiating, intergrown
crystals are common.
Vitreous to pearly luster.
Kyanite
Al2SiO5
Island
Silicates
4–5
(along
cleavage
planes)
25
COLOR
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Reddish brown
to black
Colorless
7 – 7.5
1 poor cleavage.
Often appears with
uneven fracture.
Prismatic, sometimes
stubby crystals. Crystal
surfaces frequently rough
due to weathering and
alteration. Crystals
frequently in
characteristic cruciform
shapes which are crossed
crystals @ 90o or 60o.
Less commonly they are
irregular or granular
crystals in rock. Dull,
earthy or sometimes
vitreous or resinous luster.
Staurolite
(Fe,Mg,Zn)2
(Al9Si4O23(OH)
Island
Silicates
Commonly dark
red, frequently
violet with
brownish
coloration. Other
varieties may be
green, brown,
orange, pink or
black.
Commonly white,
may vary
(difficult to
observe)
6.5 – 7.5
No cleavage.
Uneven to splintery
or subconchoidal
fracture.
Garnets are a group of
minerals including
pyrope, almandine and
grossular. Complex,
“many faced” crystal
habits that appear rounded
or spherical. Individual
crystals and dense,
granular masses of many
crystals are common.
Vitreous or adamantine
luster.
Garnet
(Mg,Fe,Ca,Cr)3
Al2(SiO4)3
Island
Silicates
26
STREAK
HARDNESS
CLEAVAGE/
FRACTURE
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
COMMON
MINERAL
ASSOC.
Colorless. The
presence of
different
impurities may
impart a wide
variety of colors
(i.e Amethyst –
purple, Smoky
quartz – black)
Colorless, white
or variable in
color.
7
No cleavage.
Commonly good
conchoidal fracture,
especially in
microcrystalline
form. Frequently has
uneven fracture.
Commonly occur as
well-defined, elongated,
hexagonal (6-sided)
crystals that may be very
large in size (amethyst).
Crystal faces may be
striated. Usually dense,
compact masses of many
poorly formed crystals or
as a microcrystalline
mass (agate, jasper).
Vitreous luster.
Microcrystalline quartz
has a dull, waxy luster.
Quartz
SiO2
Framework
Silicates
Variable
including
colorless, yellow,
blue, green,
violet or reddishyellow.
White
(difficult to
observe)
1 perfect cleavage.
Individual crystals or
crystal aggregates are the
most common form.
Prismatic crystals,
sometimes very large in
size. Striations may be
observed on crystal
faces. Vitreous luster.
Al2SiO4(F,OH)2
Island
Silicates
COLOR
8
27
and
Microcrystalline
Quartz
Topaz
COLOR
STREAK
HARDNESS
Variable color.
Commonly gray
or brown. Less
frequently red,
blue, yellow,
green, purple, or
colorless.
Colorless or white
(difficult to
observe)
9
Colorless,
yellow, brown,
gray, green or
black.
None
10
CLEAVAGE/
FRACTURE
COMMON
MINERAL
ASSOC.
OTHER
PROPERTIES
MINERAL
NAME
CHEMICAL
FORMULA
No cleavage.
Uneven or
“rectangular”
fracture.
Common form is well
formed, individual.
crystals or granular
masses. Hexagonal (6sided), stubby, barrelshaped crystals.
Adamantine luster.
Corundum
Al2O3
Oxides/
Hyroxides
4 perfect octahedral
cleavages.
Commonly roundish,
octahedral crystals.
Transparent to
adamantine luster.
Diamond
C
Native
Element
28
GEOL 1107, Lab #4
Table 4 - 3. Common minerals and their economic significance
Mineral
Andalusite
Apatite
Arsenopyrite
Azurite
Barite
Bauxite
Bornite
Calcite
Chalcopyrite
Cinnabar
Copper
Corundum
Diamond
Economic Significance
Used in the manufacture of high
temperature or acid resistant insulators and
electrical products because of its resistance
to change or reaction. Occasionally a semiprecious gem. An important index
minerals that indicates metamorphic
conditions or grade during rock
formation/alteration.
A source of phosphorous used in the
production of fertilizers and phosphoric
acid.
Economic source of arsenic, with tin, gold,
silver and cobalt as important by-products
that occur as impurities.
Valuable decorative stone. In crushed form
it has been used as a pigment. Minor
source of copper.
Economic source of barium. Used as a
high density additive in drilling mud to add
weight. Used in the manufacture of paper,
rubber, and as a radiation screen in cement.
Important economic source of aluminum.
Economic source of copper.
Used in the manufacture of building
materials (cement, ornamental stone) and
fertilizers. Widely used for metal and
chemical production as a prime ingredient
or secondary additive.
Economic source of copper, with lesser
amounts of gold and silver which are
present as impurities.
Important economic source of mercury.
Used to make the pigment vermillion.
Important component of metal alloys such
as brass and bronze. Used for electrical
wiring and coins for currency.
A semi-precious gemstone also used as an
industrial abrasive. Was used in the
manufacture of precision instruments; has
since been replaced by synthetic varieties.
Precious and semi-precious gemstones.
Industrial abrasive.
29
GEOL 1107, Lab #4
Mineral
Dolomite
Fluorite
Galena
Garnets
Gold
Graphite
Gypsum
Halite
Hematite
Kaolinite
Economic Significance
Similar to calcite. Used in the manufacture
of building materials (cement, ornamental
stone) and fertilizers. Widely used for
metal and chemical production as a prime
ingredient or secondary additive.
Used in the production of hydrofluoric acid
and as a flux in metal manufacturing.
Economic source of lead and, to a lesser
extent, silver when it’s present as an
impurity.
Semi-precious gemstones. Industrial
abrasive.
Important as a monetary standard and for
jewelry. Useful in chemistry, dentistry and
in electronic components because of its
excellent heat and electrical conductivity
and resistance to most chemical reactions.
Used in some electrical components, as a
dry lubricant, in pencils and as a dye.
Prime ingredient in plaster of Paris,
Portland cement, and some pottery.
Important fertilizer ingredient.
A very important nutrient. Halite or salt is
also used for food preparation and
preservation, and in the chemical industry
as a main ingredient in many chemical
reactions. It is also used to produce some
scientific and optical equipment and is a
source of Mg, K, Cl, Br and I.
Important economic source of iron.
Hematite in the banded iron formations of
northern Quebec are Canada’s richest iron
producing deposits. Also used as a
pigment.
A clay mineral used in the manufacture of
china dishes. A filler in the manufacture of
paper, rubber, cosmetics and medicine.
30
GEOL 1107, Lab #4
Mineral
Economic Significance
Kyanite
Used in the manufacture of high
temperature or acid resistant insulators and
electrical products because of its resistance
to change or reaction. Occasionally a semiprecious gem. An important index mineral
that indicates metamorphic conditions or
grade during rock formation/alteration.
Limonite
Magnetite
Malachite
Molybdenite
Muscovite
Plagioclase Feldspar
Potassium Feldspar
Pyrite
Pyrrhotite
Quartz
Microcrystalline Quartz
Used as a pigment. Has been, and may
continue to be, a locally significant source
of iron.
The richest, most important economic
source of iron.
Valuable decorative stone. In crushed form
it has been used as a pigment. Minor
source of copper.
Source of molybdenum which is a dry
lubricant resistant to high temperatures.
Used for electrical and heat insulation and
in the manufacture of paper, rubber,
fireproof paint and porcelain.
Used in the manufacture of ceramics.
Labradorite is a semi-precious gemstone.
Used in the manufacture of porcelain,
detergents and scouring powders.
A main ingredient in the manufacture of
sulfuric acid.
Economic source of nickel, cobalt and
platinum.
Used in the manufacture of precision
instruments (i.e. watches) because of its
piezoelectric properties, its ability to
polarize light, its transparency to ultraviolet
light, its hardness and resistance to
chemical reaction or alteration. It is also
used in the manufacture of glass, paint and
abrasives. In crystal for it is a semiprecious gemstone.
31
GEOL 1107, Lab #4
Mineral
Serpentine (Asbestos)
Sillimanite
Silver
Sphalerite
Staurolite
Talc
Topaz
Tourmaline
Vermiculite
Economic Significance
Was used for sound and thermal insulation
and in the construction of fire-resistant
materials.
Used in the manufacture of high
temperature or acid resistant insulators and
electrical products because of its resistance
to change or reaction. Occasionally a semiprecious gem. An important index mineral
that indicates metamorphic conditions or
grade during rock formation/alteration.
Used in jewelry, photographic film,
electrical components. Still has limited use
in coins for currency.
Economic source of zinc.
Semi-precious gemstones. Index mineral
used to indicate metamorphic conditions or
grade during rock formation/alteration.
Prime ingredient in paper, rubber,
cosmetics (talcum powder), and paint.
A semi-precious gemstone.
Used in the manufacture of precision
instruments. Also a semi-precious
gemstone.
When heated and expanded it is used as
insulating material. May be used as potting
soil and in the paper, paint and plastics
industries.
32
GEOL 1107, Lab #5
LABORATORY 5
IGNEOUS ROCK IDENTIFICATION
INTRODUCTION
All rocks are made up of one or more minerals. For all rocks the type of
mineral(s) and their amount, if there’s more than one, says a lot about how
the rock formed, where it formed and how long it took. Aside from the types
of minerals in a rock, the texture of the rock also indicates something about
the conditions under which it formed. All rocks are composed of either
crystals or rock fragments. Most igneous rocks are composed of crystals. The
process of crystal formation is the first thing we need to look at for igneous
rock identification.
CRYSTAL FORMATION
Mineral crystals may form by one of several different methods:
1. By sublimation which is mineral precipitation from a gas or vapour.
Frost crystals forming inside a freezer or calcite crystals from the steam
emanating from a volcanic vent are two examples.
2. By precipitation from a saturated solution. Halite or salt crystal
precipitation in water or quartz crystal precipitation in hot water
underground – also known as a hydrothermal solution – are two
common examples.
3. By the rearrangement of atoms or molecules in a pre-existing mineral.
This happens when fresh snow is compacted and converted to solid ice
or when “new” metamorphic minerals, such as sillimanite or kyanite,
form under high heat and pressure inside the Earth from a pre-existing
mineral.
4. When molten rock, such as magma or lava, cools and solidifies. This
the most common form of igneous rock formation.
Magma is molten rock beneath the Earth’s surface formed because of intense
heat within the planet. If liquid magma flows or is extruded onto the surface
of the land or seafloor, then it is called lava.
1
GEOL 1107, Lab #5
IMPORTANT NOTE
The distinction between magma and lava may seem over simplified but it
is an important consideration. Whether or not melted rock (magma or lava)
is found at or below the Earth’s surface will impact directly on the type of
rock that forms. Thus, it is a very important consideration when it comes to
rock classification.
IGNEOUS ROCKS
Igneous rocks are one of the three main classes of rock. Igneous rocks form
when magma or lava cools and solidifies at or below the Earth’s surface.
They also form when solid rock fragments, such as ash, are ejected from a
volcano and accumulate on the Earth’s surface. The rock fragments ejected
by a volcano are called pyroclastic debris.
There are two types of igneous rock:
1. Igneous intrusive rocks form when magma beneath the Earth’s
surface cools and solidifies. Igneous intrusive rocks are also called
plutonic rocks.
2. Igneous extrusive rocks form when lava cools or pyroclastic debris is
deposited on the Earth’s surface. These types of igneous rock are also
called volcanic rocks.
Igneous rocks are described, classified and identified based on two things:
composition and texture.
COMPOSITION OF IGNEOUS ROCKS
Igneous rocks are composed primarily of just a few silicate minerals, the most
common type of mineral in the Earth’s crust. The eight common silicate
minerals in the Earth’s crust and therefore the most common minerals in
igneous rocks are separated into two classes. They are:
1. Felsic
2. Mafic
2
GEOL 1107, Lab #5
Felsic minerals are lighter in colour, richer in silicon (Si), potassium (K) and
aluminum (Al), and less dense compared to mafic minerals. The most
common felsic minerals are:
1. Quartz
2. Plagioclase feldspar
3. Potassium feldspar
4. Muscovite
Mafic minerals, sometimes called ferromagnesian minerals, are darker in
colour, richer in iron (Fe) and magnesium (Mg), and more dense compared to
felsic minerals. The most common mafic minerals are:
5. Olivine
6. Pyroxene
7. Hornblende
8. Biotite
IMPORTANT NOTE
These are the most common and most important rock forming minerals you
will encounter in this course. If you take the time now to review these
minerals from labs 1, 2, 3 and 4 you will have a much easier time when it
comes to igneous, sedimentary and metamorphic rock identification.
Before we move on to texture and structure here is a quick overview of the
eight common silicate minerals to help you with their identification. The
mineral properties listed here for each mineral describe how it appears in a
rock where you may only see small crystals.
3
GEOL 1107, Lab #5
The felsic silicate minerals
1. Quartz. Commonly occurs as irregular, glassy grains typically clear
to smoky (medium light gray) in colour (Figure 5 - 1). Quartz has no
cleavage, but conchoidal fracture may be seen on some surfaces.
Quartz is different from light-coloured feldspars which appear
milkyor translucent.
Scale is in centimeters
Figure 5 - 1. Quartz
4
GEOL 1107, Lab #5
2. Muscovite. Flexible, silvery to light or white in colour, translucent
mineral flakes are commonly associated with quartz or K-feldspar
(Figure 5 - 2). Perfect cleavage in one direction. Sometimes appear as
small glittering specks.
Scale is in centimeters
Figure 5 - 2. Muscovite
5
GEOL 1107, Lab #5
3. Potassium Feldspar. Commonly pink, white, or gray coloured with
pearly, translucent to opaque luster (Figure 5 - 3). Cleavage in two
directions at right angles may be observed. Cleavage planes may
flicker or flash light as sample is rotated. Perthitic texture may be
observed with close examination.
Scale is in centimeters
Figure 5 - 3. Potassium feldspar
6
GEOL 1107, Lab #5
4. Plagioclase Feldspar. Usually gray or white when found in felsic
rocks (i. e. granite) or dark gray to bluish gray in mafic rocks (i.e.
gabbro). Pearly, translucent to opaque luster (Figure 5 - 4). Two
cleavage directions at right angles may be detected. Cleavage planes
may flicker or reflect light as the sample is rotated. Characteristic
striations may be seen with close examination.
Scale is in centimeters
Figure 5 - 4. Plagioclase feldspar
7
GEOL 1107, Lab #5
The mafic silicate minerals
5. Biotite. Flexible, black, translucent to opaque, shiny mineral flakes
(Figure 5 - 5). Perfect cleavage in one direction. When mineral
crystals are small it may be confused with hornblende or pyroxene.
Cleavage and hardness help distinguish biotite from others. Biotite
can be scratched or crushed with a steel probe.
Scale is in centimeters
Figure 5 - 5. Biotite
8
GEOL 1107, Lab #5
6. Hornblende. Long, needle-like or acicular black crystals (Figure 5 - 6).
Harder than biotite. Not as shiny or glassy as biotite. Opaque. Two
good cleavages at cleavages at 600 and 1200.
Scale is in centimeters
Figure 5 - 6. Hornblende
9
GEOL 1107, Lab #5
7. Pyroxene. Shorter, rounder, more spherical crystals, greenish black
to black in colour, with a duller, less shiny, resinous luster (Figure 5 7). Opaque to translucent. Two good cleavage directions at 900.
Commonly associated with darker mafic rocks (i.e. pyroxenite,
peridotite). Less common in felsic rocks (i.e. granitic rocks). If
mineral crystal size is very fine, it may be difficult to distinguish
between pyroxene and hornblende. It is therefore appropriate to put
“presence of pyroxene and/or hornblende” or “presence of mafic
minerals“ in your description.
Scale is in centimeters
Figure 5 - 7. Pyroxene
10
GEOL 1107, Lab #5
8. Olivine. Glassy, light green, occasionally yellow or light orange,
short stubby crystals massed together (Figure 5 - 8). One poor
cleavage. Conchoidal fracture on some grains. Clusters of many
olivine crystals have the appearance of “green sugar”.
Scale is in centimeters
Figure 5 - 8. Olivine
11
GEOL 1107, Lab #5
After you understand that the common igneous rocks are made up of the
eight common silicate minerals, the second thing you need to know for
igneous rock identification is there are four different types of rock and that
each type is made up of different amounts of the eight common silicate
minerals. The four type of igneous rock are:
1. Felsic rocks. They are composed of:
10 to 40 % Quartz
30 to 60 % Potassium Feldspar
0 to 30 % Plagioclase Feldspar
10 to 30 % Biotite and Hornblende
Magmas that produce felsic rocks are high in potassium, silicon, and
sodium and low in iron, magnesium and calcium. Typical felsic rocks
include the intrusive “granitic” rocks such as quartz monzonite,
granodiorites and igneous extrusive rocks such as rhyolite and dacite. All
are generally light in colour.
2. Intermediate rocks. These rocks are composed of:
25 to 45% Hornblende and Biotite
55 to 75 % Plagioclase Feldspar
K - feldspar and quartz are present in minor amounts only. Intermediate
type rocks, such as andesite and diorite are “intermediate” in composition
between the felsic and the mafic rocks. They are characteristically gray in
colour.
3. Mafic rocks. They are composed of:
45 to 75 % Ca-rich Plagioclase Feldspar
25 to 55 % mix of Olivine, Pyroxene and Hornblende
Mafic rocks crystallize from magmas that are relatively high in iron,
magnesium and calcium, and low in silica. Rock colour is typically black or
dark green.
12
GEOL 1107, Lab #5
4. Ultramafic rocks. They are made up of:
10 to 100% Olivine
0 to 90% Pyroxene
0 to 5% Ca-rich Plagioclase Feldspar
Ultramafic rocks such as dunite, peridotite and pyroxenite are relatively
rare compared to other rock types. They are composed almost exclusively
of mafic minerals, which impart a black colour. They form under special
conditions that are usually found deep inside the Earth.
TEXTURE AND STRUCTURE
Texture and structure are the other important aspects of igneous rocks used
for identification. The texture of a rock, sometimes called the fabric, is the
size, shape and arrangement of mineral crystals or rock fragments in the rock.
Structure is the way in which mineral crystals and rock fragments are
arranged. Structure refers to larger features such as layers.
There are a number of factors that may affect texture and structure of an
igneous rock. The size of mineral crystals that form when magma or lava
cools and solidifies is dependent on:
1. The rate of crystallization or crystal growth as magma or lava cools
and solidifies. Igneous extrusive or volcanic rocks, which solidify
quickly in a cooler environment at the Earth’s surface, tend to have
smaller or finer crystals. Igneous intrusive or plutonic rocks, which
crystallize slowly in a well-insulated, hot environment deep within
the Earth’s crust, tend to have large or medium to coarse grains or
crystals.
2. The presence of volatile fluids (mainly water) dissolved in magma
or lava will help promote crystallization. Magmas, for example, can
only retain dissolved fluids, such as water, under conditions of high
pressure which occurs when the magma is buried deep inside the
Earth. When magma rises towards the surface pressure is reduced
and dissolved fluids are released. The effects of fluids and changing
pressure mostly affects igneous intrusive rocks. They do not
generally affect volcanic rocks.
13
GEOL 1107, Lab #5
3. The viscosity of the magma. Viscosity is a measure of a fluids ability
to flow, in this case a magma or lava. A fluid with a high viscosity
cannot easily flow. For example, molasses kept inside a refrigerator
has a high viscosity. A fluid with a low viscosity, such as liquid
water at room temperature, flows more easily.
Viscosity affects crystal growth rate. Felsic magma is much more
viscous than a mafic magma. As a result, mineral crystals have a
harder time growing in a felsic magma than they do in a mafic
magma. For this reason, felsic igneous rocks tend to have smaller or
finer crystals. Microcrystalline textures, where crystals are not visible
to the unaided eye, and glassy textures, where minerals in the rock
have little or no crystalline texture, are found exclusively in igneous
extrusive rocks. Mafic minerals in igneous rocks maybe tend to have
better developed crystal structures regardless of crystal size.
Texture is an important part of igneous rock identification. Common igneous
rock textures include:
1. Glassy. If lava cools very rapidly, such as when it encounters sea
water or glacial ice, there is no time for the atoms in the melt to form
a crystalline structure (Figure 5 - 9). The result is an amorphous,
glassy rock called obsidian.
Scale is in centimeters
Figure 5 - 9. The glassy texture of the rock obsidian
14
GEOL 1107, Lab #5
2. Aphanitic. A fine crystalline texture forms when lava or magma
cools slowly enough for mineral crystals to form, but still fast enough
that only small crystals have time to grow (Figure 5 - 10). Aphanitic
rocks are those in which the mineral crystals are smaller than 1 mm
and therefore difficult or impossible to see without the aid of a hand
lens or microscope. An aphanitic texture usually indicates that the
rock has undergone rapid cooling at the Earth’s surface and as such
gives some insight into the history of the rock. Aphanitic texture is
found only in igneous extrusive rocks such as basalt.
Scale is in centimeters
Figure 5 - 10. The aphanitic texture of the rock basalt
15
GEOL 1107, Lab #5
3. Phaneritic. Igneous intrusive rocks have coarse or large visible
crystals. The term phaneritic comes from the Greek word for
“visible”. A rock with a phaneritic texture has mineral crystals
between 1 and 10 mm in size (Figure 5 - 11). They form when magma
cools more slowly beneath the Earth’s surface. Quartz diorite is the
igneous intrusive rock that makes up the bedrock of the Coast
Mountains in North Vancouver (Figure 5 - 12). It has a phaneritic
texture.
Scale is in centimeters
Figure 5 - 11. The phaneritic texture of the rock gabbro
16
GEOL 1107, Lab #5
Figure 5 - 12. The phaneritic texture of the rock quartz diorite
Plagioclase
Feldspar
Quartz
Mafic
Minerals
Potassium
Feldspar
Figure 5 - 13. The phaneritic texture of the rock true granite showing quartz, potassium
feldspar, plagioclase feldspar and mafic minerals together in an igneous intrusive rock.
17
GEOL 1107, Lab #5
4. Porphyritic. This term refers to igneous rocks that have two distinct sizes
of mineral crystals, large and small (Figure 5 - 14). The presence of two
distinct crystal sizes reflects a two part history of formation: 1) First slow
cooling of magma to form large crystals called phenocrysts, followed by 2)
faster cooling and solidification of the rest of the magma to form a matrix
or groundmass of small or fine that surrounds and holds in place the
larger phenocrysts. Common extrusive igneous rocks with this so-called
bi-modal or two part texture include dacite, trachyte, rhyolite and
andesite.
Gray Groundmass
Large Black
Mafic Crystals
Scale is in centimeters
Figure 5 - 14. The porphyritic texture of the rock andesite
18
GEOL 1107, Lab #5
5. Pegmatitic. This rock texture is made up of large mineral crystals
(Figure 5 - 15). They form when magma cools and solidifies very
slowly, perhaps because of a high concentration of volatile material
(i.e. water, dissolved gas) or when all or part of a magma starts to
crystallize at low temperature and, as a result, is much cooler or more
concentrated. Either way, a body of magma collects underground
which, when completely crystallized or solidified, is made up of
potentially very large crystals. The average crystal size in a
pegmatitic texture is greater than 1 cm. Some pegmatites contain
minerals over a meter in size. A pegmatite is an igneous intrusive
rock with large (> 1 cm) crystals. Identification of a pegmatite is
basedsolely on texture. Mineral composition is not a factor.
Scale is in centimeters
Figure 5 - 15. The very coarse crystalline texture of the rock pegmatite
19
GEOL 1107, Lab #5
6. Pyroclastic or Fragmental. A pyroclastic or fragmented texture is
found exclusively in volcanic rock fragments or pyroclastic debris
ejected by a volcano and deposited on the Earth’s surface. Deposition
is similar to sediment and sedimentary rock deposition. The term
pyroclastic is derived from the Greek word for “fire broken”.
Classification of pyroclastic debris is based on the size of the
fragments. Ash size fragments are less than 2mm. Lapilli includes
fragments 2 to 64 mm. Volcanic bombs are greater than 64 mm in
size.
Pyroclastic igneous rocks generally fall into one of two classes. They
are: 1) a tuff which is a lithified volcanic ash (Figure 5 - 16), and 2)
volcanic breccia (Figure 5 - 17) which is made up of larger lapilli
and/or volcanic bomb-size fragments.
Scale is in centimeters
Figure 5 - 16. The small, ash-size fragments of a pyroclastic or
fragmental texture in an ashflow tuff
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GEOL 1107, Lab #5
Scale is in centimeters
Figure 5 - 17. The pyroclastic or fragmental texture of a volcanic breccia
with large angular fragments
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GEOL 1107, Lab #5
7. Vesicular. This is a texture found only in cooled and solidified lava.
It is made up of small or fine crystals and many holes, empty spaces
or vesicles formed by the inclusion of gas bubbles when the lava
quickly cooled (Figure 5 - 18). The end result is a very porous or
vesicular rock with an aphanitic mineral texture. Vesicular texture is
often associated with basalt. Pumice is an igneous extrusive rock
with a vesicular texture that is so porous it will float on water (Figure
5 - 19). This is because there was so much trapped volcanic gas.
Figure 5 - 18. The vesicular texture of basalt
Figure 5 - 19. The vesicular texture of pumice
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GEOL 1107, Lab #5
8. Amygdaloidal. This texture begins with vesicles or a vesicular
texture. After they form the holes or pores are completely or partially
filled with a secondary mineral precipitate (Figure 5 - 20). The walls
of a vesicle or pore may be covered or lined with very fine grain
calcite or quartz which has precipitated in this void space from
solutions circulating through the rock.
Vesicle
Dark gray
aphanitic or
small crystals
of basalt
Vesicle filled with white
secondary mineral
Scale is in centimeters
Figure 5 - 20. The amygdaloidal texture of a vesicular basalt. This basalt is
made up of fine crystals, so it has an aphanitic texture. There are also vesicles.
Some of these vesicles are filled with a white, secondary mineral giving it an
amygdaloidal texture.
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GEOL 1107, Lab #5
ESTIMATING THE AMOUNT OF MINERALS IN A ROCK
Once the mineralogy, texture and structure of an igneous rock have been
identified, the next step is to determine the relative abundance of each
mineral. For example, if quartz, k-feldspar and mafic minerals are the only
minerals in the rock you must estimate what percentage of the total volume
of the rock (100%) is quartz, k-feldspar and mafic. Estimating percentages
can be a tricky procedure fraught with uncertainty and error. However, a
reasonable approximation (+/- 10%) is usually good enough to start with. To
help you begin there are a series of diagrams on pages 25 to 28 in this lab.
The black dots or shapes in each of the circles amount to a percentage of the
total area of the circle. Use these to help estimate the amount of different
minerals in a rock sample.
The diagrams on pages 25 to 28 are taken from Howell Williams, Francis J.
Turner, Charles M. Gilber, 1982. Petrography: An Introduction to the Study of
Rocks in Thin Section. W. H. Freeman, p. 593.
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GEOL 1107, Lab #5
Estimating mineral abundance in a rock sample, 1%, 2% and 3%.
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GEOL 1107, Lab #5
Estimating mineral abundance in a rock sample, 5%, 7% and 10%.
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GEOL 1107, Lab #5
Estimating mineral abundance in a rock sample, 15%, 20% and 25%.
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GEOL 1107, Lab #5
Estimating mineral abundance in a rock sample, 30%, 40% and 50%.
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GEOL 1107, Lab #5
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GEOL 1107, Lab #5
Figure 5 -21 (Page 29). For the common igneous rocks, a system of classification is presented in
table form. This includes the common igneous extrusive and intrusive rock types. The top half
of the chart is for identifying igneous extrusive rocks. The bottom half of the chart is used to
identify intrusive igneous rocks.
PROCEDURE FOR IGNEOUS ROCK IDENTIFICATION
Now that you have all the tools for igneous rock identification, the next thing is to
gather all the information and come up with a name for the rock. What follows is a six
step procedure you can use for igneous rock identification. This procedure is a guide.
Use itif you want, modify it if need be. All we want you to do is identify the rock and
tell us why you gave it that name. How you do it is up to you.
STEP 1
The first step is to determine the texture of the rock.
Questions and Answers
You need to decide which of the following five textures the rock has.
1. Does the rock have a pyroclastic or fragmental (rock fragments) texture?
If the rock texture is pyroclastic or fragmental it is an igneous extrusive or volcanic rock.
This type of rock is found in the top part of the igneous rock identification chart (Figure
5 - 21 on page 29). Pyroclastic or fragmental rocks include volcanic breccias, volcanic
conglomerates, and three tuffs - ashfall tuff, ashflow tuff and welded tuff. A breccia
is made up of large, angular rock fragments, on average more than 2 mm in size. A
conglomerate has large, rounded rock fragments and is quite rare. A tuff is a volcanic
rock composed almost entirely of fine-grained, volcanic ash. Ash is very small rock
fragments less than 1 mm in size.
If the rock sample has a pyroclastic or fragmental texture go to step 2.
2. Does the rock have a glassy texture?
If the rock texture is glassy then the rock is one of two igneous extrusive rocks: obsidian
or pumice. Based on a more detailed understanding of glassy texture you decide what the
rock is, an obsidian or a pumice and the rock is identified. You are done.
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GEOL 1107, Lab #5
3. Does the rock have a porphyritic texture or an aphanitic texture? Is it made up of
large and small crystals or is it made up of only small crystals?
If the rock has an aphanitic or porphyritic texture it is an igneous extrusive rock. The
name of a rock with this texture is found in the middle of the igneous rock identification
chart (Figure 5 - 21 on page 29). It is either a felsite, trachyte, rhyolite, dacite, andesite
or a basalt.
If the rock sample has an aphanitic or porphyritic texture go to step 3.
4. Does the rock have a phaneritic texture? Is it made up entirely of large crystals?
If the rock is made up of large crystals or has a phaneritic texture it is an igneous
intrusive rock and is found in the bottom part of the igneous identification chart where
there are ten possible answers from syenite through to dunite.
If the rock sample has a phaneritic texture go to step 4.
STEP 2
This step is where you identify a rock that has a pyroclastic or fragmental texture.
Questions and Answers
If the rock has a pyroclastic or fragmental texture answer the following questions. If the
answer is yes, then you have found the name of the rock. If the answer is no, then go to
the next question.
1. Are the rock fragments large or coarse (on average > 2 mm) and angular? If yes,
then the rock is a volcanic breccia.
2. Are the rock fragments large or coarse (on average > 2 mm) and rounded? If
yes, then the rock is a volcanic conglomerate.
3. Is the rock composed entirely of well sorted, small or fine, consolidated rock
fragments (ash)? If yes, then the rock is an ashfall tuff.
4. Is the rock composed of a mix of poorly sorted medium and fine, consolidated
rock fragments (ash)? If yes then the rock is an ashflow tuff.
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GEOL 1107, Lab #5
5. Is the rock composed of well to poorly sorted, mostly fine grain, consolidated
rock fragments (ash) and is there layering or banding in the rock? If yes, then
the rock is a welded tuff.
STEP 3
This step is what to do if the rock texture is aphanitic or porphyritic.
At this point you’ve determined the rock has either an aphanitic or a porphyritic
texture. The next thing to do is determine which texture is it by answering questions 1
and 2 in this step.
Question 1
1. Does the rock have an aphanitic texture? If yes go to the next sentence. If not go to
question 2.
If the rock has an aphanitic texture the next property is colour. Individual mineral
crystals in a rock with an aphanitic texture are generally too small to see so it’s very
difficult to see individual mineral colours. Fortunately, in most cases, a rock with this
texture is all one colour.
If the rock is all white or light coloured and it has an aphanitic texture then it is a felsite.
If the rock is all black or dark coloured and it has an aphanitic texture then it is a basalt.
If the rock has a colour intermediate between light and dark, often this means a gray
colour, and it has an aphanitic texture then it is an andesite.
IMPORTANT NOTE
Rock identification is based in part on mineral content. However, in many rocks you
will find individual mineral crystals that are too small to properly identify. Do the
best you can, this is a problem even experienced geologists encounter. The most
reliable mineral indicator, when the sample is small and the crystals indistinct, is
colour.
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GEOL 1107, Lab #5
In the procedure so far, this lab refers to “dark or black” coloured (mafic) minerals
and “light or white” coloured (felsic) minerals. Generally speaking, a dark mineral
refers to anything that is black or dark gray. Light or white coloured refers to all the
remaining minerals such as white, clear, pink or orange.
Question 2
2. Does the rock have a porphyritic texture? If yes, go to the next sentence.
If the rock has a porphyritic texture this means it has two distinct sizes of crystals, large
and small. The next step is to determine the mineralogy of the large crystals, the ones
you can see. Identifying the large crystals is pretty straight forward because we only
need to decide which of the eight common silicate minerals they are. The rest of the
rock, the fine crystalline groundmass is only used to identify an andesite and a basalt.
If the large crystals in the rock are pink or orange, then they are potassium feldspar. If
this is the case, then the rock is a trachyte.
If the large crystals in the rock are a mix of pink or orange crystals and crystals that are
clear and look like a piece of glass, then they are potassium feldspar and quartz. If this
is the case, then the rock is a rhyolite.
If the large crystals in the rock are a mix of clear glassy and white crystals, then they are
quartz and plagioclase feldspar. If this is what’s in the rock, then it’s a dacite.
If the large crystals in the rock are a mix of white and black crystals, then the rock
contains plagioclase feldspar and mafic minerals. It is not necessary to distinguish
what type of mafic minerals there are. Calling all black minerals mafic is good enough.
If the large minerals are a mix of plagioclase feldspar and mafic minerals you have a
choice of two rocks: andesite or basalt. Here’s how you tell the difference: If the
colour of the groundmass (the fine crystals surrounding the large crystals) in this
porphyritic texture is any colour but black then the rock is an andesite. If the
groundmass is black, then the rock is a basalt.
STEP 4
This step is what to do if the rock has a phaneritic texture.
Now that you’ve decided you have a rock with a phaneritic texture, which is a rock
made up entirely of large crystals, the next step is to identify what the minerals are.
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GEOL 1107, Lab #5
The most reliable way to determine the types of minerals in an igneous rock is colour.
Mineral properties like cleavage, streak and hardness are of limited use when the mineral
is part of rock and can’t be examined in the ways that a separate mineral sample can.
The most common minerals in igneous intrusive rocks are the eight common silicate
minerals. To identify these minerals in a rock all you need to know are four colours.
Mineral identification for igneous intrusive rocks works like this:
1. If the large crystals in the rock are pink or orange, then they are
potassium feldspar.
2. If the large crystals in the rock are white, then they are plagioclase feldspar.
3. If the large crystals in the rock are clear and look like a piece of glass, then
they‘re quartz.
4. If the large crystals are black, then they are called mafic. It is possible to
identifythe different mafic minerals - olivine, pyroxene, hornblende and
biotite - using the information in lab 4 and the descriptions on pages 4 to 11 in
this lab but it’s not absolutely necessary. In many cases of rock identification, it
is enough to say all the black minerals are mafic.
Keep in mind that only the eight common silicate minerals are used to identify the
common igneous intrusive rocks. It’s also important to know that igneous intrusive
rocks contain only a few - one, two, three or four - of the eight common silicate
minerals.
Once you’ve identified all the minerals in the rock sample go to step 5.
STEP 5
This step is where you estimate the percentage of each mineral in a rock.
Now that you’ve identified the rock texture as phaneritic and the minerals in it, the next
step is to estimate how much of the rock is composed of the different minerals. Use the
mineral estimation diagrams on pages 25 to 28 to help with this.
All you need to do is ask yourself the following questions:
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GEOL 1107, Lab #5
1. What percentage of the rock is potassium feldspar which are the large pink or
orange crystals?
2. What percentage of the rock is plagioclase feldspar, the large white crystals?
3. What percentage of the rock is quartz which are the large clear or glassy
looking crystals?
4. What percentage of the rock are the mafic minerals which are the large black
crystals? It may be possible to identify olivine, pyroxene, hornblende and
biotite but it’s not a requirement.
Make sure the percentages for each mineral when added together equal 100%. Don’t
give a range for a value, in other words don’t say the rock contains 20 to 30%
plagioclase feldspar. It you think the estimate varies then choose a single number that
is an average. For example instead of a range of 20 to 30% record your estimate as 25%.
Once you have all your estimates of mineral percentages go to step 6.
STEP 6
Name that rock.
Now you have all the information needed to identify an igneous intrusive rock with a
phaneritic texture. This final step is to select the proper name from the identification
chart (Figure 5 - 21 on page 29). The bottom half of this chart has the names of the ten
common igneous intrusive rocks with a phaneritic texture and information about their
mineralogy. The mineralogy of the ten common rocks is illustrated in the chart and
explained in the following way:
1. Syenite is made up of:
0% to 10% quartz
65% to 85% potassium feldspar
10% to 35% hornblende and biotite
No plagioclase feldspar
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GEOL 1107, Lab #5
2. True Granite is made up of:
10% to 38% quartz
48% to 80% potassium feldspar
0% to 7% plagioclase feldspar
5% to 10% hornblende and biotite
3. Quartz Monzonite is made up of:
32% to 38% quartz
25% to 48% potassium feldspar
7% to 25% plagioclase feldspar
10% to 20% hornblende and biotite
4. Granodiorite is made up of:
20% to 32% quartz
10% to 25% potassium feldspar
25% to 45% plagioclase feldspar
20% to 25% hornblende and biotite
5. Quartz Diorite is made up of:
10% to 20% quartz
0% to 10% potassium feldspar
45% to 60% plagioclase feldspar
25% to 30% hornblende and biotite
0% and 5% pyroxene
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GEOL 1107, Lab #5
6. Diorite is made up of:
0% to 10% quartz
No potassium feldspar
60% plagioclase feldspar
10% to 25% hornblende and biotite
5% to 30% pyroxene
7. Gabbro is made up of:
No quartz
No potassium feldspar
10% to 60% plagioclase feldspar
No hornblende and biotite
30% to 75% pyroxene
0 to 15% olivine
8. Pyroxenite is made up of:
No quartz
No potassium feldspar
10% to 0% plagioclase feldspar
75% to 90% pyroxene
5% to 20% olivine
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GEOL 1107, Lab #5
9. Peridotite is made up of:
No quartz
No potassium feldspar
No plagioclase feldspar
20% to 80% pyroxene
20% to 80% olivine
10. Dunite is made up of:
No quartz
No potassium feldspar
No plagioclase feldspar
0% to 20% pyroxene
80% to 100% olivine
The identification of a rock is done by comparing the percentages of minerals in the
unknown rock with the percentages of minerals for each of the ten rock types listed
above and in figure 5 - 21 on page 29.
Now let’s go through a couple of examples.
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GEOL 1107, Lab #5
Example 1
Scale is in centimeters
Figure 5 - 22. Example 2
Figure 5 - 22 is an image of an igneous intrusive rock made up entirely of large crystals.
The large crystals are orange, black, white and clear glassy in colour. These four colours
are the maximum number of colours in an igneous rock. The orange minerals are
potassium feldspar, and the white minerals are plagioclase feldspar. The black minerals
are mafic. Again, we’re not concerned with what type of mafic minerals they are. The
clear glassy minerals are quartz. In this second example it appears that all the common
silicate minerals are present. Now for the percentage of each mineral. Here’s what the
estimates are:
Potassium feldspar is 30%.
Plagioclase feldspar is 20%.
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GEOL 1107, Lab #5
Mafic minerals are 15%.
Quartz is 35%
Now let’s determine which of the ten possible answers this rock is not. It’s not syenite
or true granite because both these rocks contain little or no plagioclase feldspar and we
have 20% in example 2. Quartz diorite, diorite, gabbro, pyroxenite, peridotite and
dunite are all eliminated because these rocks contain little or no potassium feldspar and
our sample has 30%.
This then leaves , quartz monzonite and granodiorite as possible rock names.
A quartz monzonite is made up of:
The more common of the
two feldspars in this rock
32% to 38% quartz
25% to 48% potassium feldspar
7% to 25% plagioclase feldspar
10% to 20% hornblende and biotite
A granodiorite has:
20% to 30% quartz
The more common of the
two feldspars in this rock
10% to 25% potassium feldspar
25% to 45% plagioclase feldspar
20% to 25% hornblende and biotite
So, what’s an important difference between quartz monzonite and a granodiorite? The
quartz monzonite has more potassium feldspar than the plagioclase feldspar while the
granodiorite has more plagioclase feldspar than potassium feldspar. Example 2 has
more potassium feldspar (30%) than plagioclase feldspar (20%) and so it is a quartz
monzonite.
Could example 1 be a granodiorite? It could. If instead the percentages of feldspar
minerals were estimated differently, say 20% potassium feldspar and 30% plagioclase
feldspar, then example 1 would be a granodiorite. Estimating mineral percentages is
the part of rock identification with the most uncertainty. Approximate values for
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GEOL 1107, Lab #5
percentage, not exact values, will work.
A VERY IMPORTANT NOTE
Estimating the percentage of each mineral in a rock is the hardest thing to do because
it is an estimate, it is a number based on your ability to observe a rock and compare
what you see to the diagrams on page 25 to 28. It’s entirely possible that one person
will have percent values different from someone else. If this is the case, it’s still
possible for each person to arrive at a correct answer. Take the rock in example 1.
Two possible answers were given: diorite and gabbro. Each is correct based on the
percentage of plagioclase feldspar and mafic minerals. So, which is correct? Both
rocks are very similar in appearance, sharing the same compositions and textures.
Depending upon the sample, both answers are acceptable, one may, however, be
more right than the other. The relative abundance of minerals will look different to
different people and it will vary from sample to sample, even if they’re from the
same rock deposit. Rocks are natural objects and, like everything else in nature, they
will vary in appearance and composition. Remember, estimating mineral
percentages is not an exact science.
So, when it comes to igneous intrusive rock identification make sure you describe the
texture, mineralogy, percentage of each mineral and use this information to name the
rock. If your answer is different from someone else make sure you base your
decision on what you see.
A final point on igneous rock identification. Don’t forget about pegmatites.
A pegmatite is an igneous intrusive rock with large (> 1 cm) crystals (Figure 5 - 15, page
19). Identification of a pegmatite is based solely on texture. Mineral composition is nota
factor.
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GEOL 1107, Lab #5
EXERCISE 5 - 1. MINERAL IDENTIFICATION: REVIEW
The most important igneous minerals are listed below and examples are provided in the
lab. Match the sample number with the names listed below:
1. Felsic minerals
i)
Quartz
ii) Potassium feldspar
iii) Plagioclase feldspar
iv) Muscovite
2. Mafic Minerals
i)
Olivine
ii) Pyroxene
iii) Hornblende
iv) Biotite
EXERCISE 5 -2: TEXTURE AND STRUCTURE OF IGNEOUS ROCKS
Examine the igneous rock texture display in the lab. These are examples of the texture
types described on pages 14 to 23 (Figures 5 - 14 to 5 - 23) in this lab. Go through the list
below, re-familiarize yourself with the descriptions and ensure you know where these
textures appear in the rock classification chart on page 29.
For each type of textural find the number of the sample that represents it. Also, in the
space provided, briefly describe some of the geological process or events that would
account for the formation of a particular texture or structure.
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GEOL 1107, Lab #5
i) Vesicular structure
No.
ii) Amygdaloidal structure
No.
iii) Aphanitic texture
No.
iv) Phaneritic texture
No.
v) Porphyritic texture
No.
vi) Fragmental texture
No.
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GEOL 1107, Lab #5
EXERCISE 5 - 3: ROCK IDENTIFICATION, IGNEOUS INTRUSIVE ROCKS
Look at the large rock samples at the front of the lab.
1. Identify 3 minerals in rock “a” and indicate the percentage of each:
i)
ii)
iii)
Note the difference between feldspar and quartz in this sample.
Both are relatively easy to distinguish.
2. What type of feldspar is in rock “a” ?
3. How can you tell what type of feldspar it is?
4. Name rock “a”.
5. Identify 3 minerals in rock ‘1’ and indicate the percentage of each.
i)
ii)
iii)
6. What type of feldspar is in rock “1” ?
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GEOL 1107, Lab #5
7. How can you tell what type of feldspar it is?
8. Name rock “1”. (hint: note the grain or crystal size)
9. Identify 2 minerals in rock ‘b’. Estimating the percentage of each mineral.
i)
ii)
10. Name rock “b”.
11. What minerals are present in the orbicular structures in rock R21?
i)
ii)
12. Draw a sketch showing the distribution of minerals in the orbicular structures.
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GEOL 1107, Lab #5
13. Identify the white phenocryst mineral in rock R9c which is found in your igneous
rock box.
14. What other minerals can you identify in rock R9c?
EXERCISE 5 - 4: ROCK IDENTIFICATION, IGNEOUS EXTRUSIVE ROCKS
1. Identify two phenocryst minerals in rock R6.
i)
ii)
2. Identify rock R6.
While you have rock R6 compare its porphyritic texture with the equigranular
texture of the plutonic rocks in exercise 5-3.
3. Name the mineralogy of the glassy phenocryst in this “snowflake” obsidian, rock
R50a.
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GEOL 1107, Lab #5
4. Examine tuff samples A and B. Handle them with care, do not scratch or dig into the
samples. Describe the sizes and shapes of the pyroclastic material in these samples.
5. Examine the vesicular lava in rocks samples C and D. Note the flow band especially
in the lighter-coloured sample. Describe the appearance of the sample under
magnification.
6. Examine ash samples E (Drinkwater) and F (Bridge River). Do you think these ash
materials formed from cold, solid material shattered by an eruption or molten lava
solidified as it was sprayed into the atmosphere? (Hint: Use the microscope to see
more closely the texture of the two ash samples).
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GEOL 1107, Lab #5
7. Examine the volcanic bomb, sample G. Sketch the general shape of the sample and
briefly explain its origin.
8. Examine the lava samples, H and I. Observe the texture of the “aa” sample and
compare it to the texture of the pahoehoe lava. Make sure you can distinguish
between these two types of rock.
9. Using the rock identification chart (page 29) determine the rock type in samples H
and I.
EXERCISE 5 - 5: ROCK IDENTIFICATION
Identify all the rocks in the box of igneous rocks. In this box there are both intrusive
and extrusive rocks. Put your answers on the next page in the appropriate blank space.
Answers should include a description of texture, different types of minerals, structures
if any, and, for igneous intrusive rocks, the abundance of the minerals expressed as a
percentage.
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GEOL 1107, Lab #5
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GEOL 1107, Lab #6
LABORATORY 6
SEDIMENTARY ROCK IDENTIFICATION
INTRODUCTION
TYPES OF SEDIMENTARY ROCK
There are two types of sedimentary rocks: 1. clastic and 2. chemical. Each records different
rock forming processes and environments.
Clastic Sedimentary Rocks
Clastic sedimentary rocks are derived from particles, pieces or clasts of minerals, rocks or
organic matter that have been separated from their parent material by processes of chemical
and physical weathering. Clasts are then eroded and transported by moving air, water, ice
or gravity, during which they may undergo significant modification to their size and shape.
Eventually all clasts are deposited when the energy of transport is lost or dissipated.
Common examples of clastic sedimentary rocks include breccias, conglomerates,
sandstones and mudstones.
Chemical Sedimentary Rocks
Chemical sedimentary rocks are precipitated from the products of chemical weathering
reactions which produce dissolved atoms and molecules, mostly found in water. Minerals
precipitate when either the temperature or volume of water drops, and dissolved atoms and
molecules can no longer be kept in solution. The most common forms of chemical
sedimentary rock are limestone, dolomite and the evaporites (i.e. salt, gypsum).
Another common form of chemical sedimentary rocks are those produced by biogenic
processes such as the metabolic activity of marine invertebrates (i.e. marine or freshwater
mollusks that form hard calcite outer shells). The remains of biogenic shells accumulate,
often as re-worked clasts to form important sedimentary rock deposits such as the
limestone in carbonate reefs. Common biogenic chemical sedimentary rocks include almost
all the limestones.
Another form of chemical sedimentary rock, sometimes called an organic sedimentary rock,
is coal. Coal is formed from the partially preserved remains of terrestrial or land plants (i.e.
trees, grasses and sphagnum).
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GEOL 1107, Lab #6
The processes responsible for the formation of clastic and chemical sedimentary rocks are:
1. Weathering. The process by which minerals, rocks and organic material are broken
down into loose fragments or clasts or dissolved in water by physical or chemical
processes.
2. Erosion. The loosening and subsequent removal of sediment from one place so it
can be transported to another. This is how sediment is set in motion.
3. Transportation. How sediment moves from one place to another by moving air,
water, and ice or by gravity.
4. Deposition. When moving sediment is dropped or otherwise placed back on the
Earth’s surface when it can no longer be transported, usually because of a drop in
the velocity of water and air, melting of the ice, or because falling sediment reaches
the bottom of a slope and gravity no longer is able to move it.
5. Precipitation. When a solution, which in nature is most commonly water, becomes
over or super-saturated with dissolved atoms and molecules. The water, because of
a decrease in volume or temperature can no longer hold these atoms or molecules
in solution and they combine to form a solid precipitate.
6. Lithification. The process by which loose, unconsolidated sediment is compressed
or glued together to form a solid, coherent rock.
The normal sequence of events in the formation of a sedimentary rock, from start to finish,
is:
1. Weathering
2. Erosion
3. Transport
4. Deposition and / or precipitation
5. Lithification
The best way to examine and classify sedimentary rocks is to take the same approach used
when examining igneous rocks: examine their texture and composition. The first group of
sedimentary rocks to look at is the clastic sedimentary rocks.
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GEOL 1107, Lab #6
CLASTIC SEDIMENTARY ROCKS
TEXTURE
The texture of a clastic sedimentary rock is defined by the size of individual clasts, their
shape - which includes roundness and sphericity - and how well sorted they are. These
same features are used to identify a clastic sedimentary rock. Texture provides important
clues as to the distance particles have been transported since weathering and erosion, what
conditions they were transported under and finally in what kind of environment they were
eventually deposited in.
1. Clast Size
The size of grains, particles or clasts ranges from large blocks several meters in diameter to
fine dust only a fraction of a millimeter in size. The size distribution of clasts is a good
indication of energy levels (i.e. how fast was the water moving) during erosion, transport
and deposition.
A simple classification system based on clast size for clastic sedimentary sediment and the
rocks that form from lithified sediment is describe here and summarized in figure 6 - 1.
Coarse grained sedimentary rocks are those in which the average clast size is > 2 mm in
diameter. This includes all the gravel-size material (granules, pebbles, cobbles and
boulders). Rocks which contain clasts in this size category are called conglomerates
(clasts are rounded) or breccias (clasts are angular) and sometimes rudites. They are
normally deposited in high energy environments.
Medium grained sedimentary rocks are those in which the average clast size is
between 0.0625 mm to 2 mm diameter which is sand-size material. Rocks which
contain clasts in this size category are called sandstones or arenites and normally
accumulate in environments with moderate energy levels.
Fine grained sedimentary rocks include those with an average clast size < 0.0625 mm
in diameter which are the silt (0.002 mm to 0.0625 mm) and clay-size (< 0.002 mm)
clasts. Rocks with an abundance of particles in this size category are called
mudstones, shales, siltstones, claystones or, less commonly, a lutite. They suggest an
environment of deposition characterized by very low levels of energy.
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GEOL 1107, Lab #6
Figure 6 - 1. Size classification of clastic sediment and the type of common clastic
sedimentary rock associated with each class.
2. Roundness
Roundness refers to the smoothness of the surface of the clast. The roundness of a
sedimentary clast is a product of, and a direct reflection of, the intensity and duration of
abrasion that the clast has undergone during transport. For example, the surface of sand
clast will become smooth or more rounded the longer or more intense the transport.
Almost all sediment, when it is first weathered and eroded from its parent material, is
angular, which means it has sharp, protruding edges. Sediment moved by ice or that falls
because of gravity is more likely to remain angular because the intensity or duration of
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GEOL 1107, Lab #6
transport is short. Particles carried by wind or water are more likely to be rounded because
abrasion is more intense and longer lasting.
The roundness of a particular clast may indicate some of its recent history. A large, angular
boulder, for example, indicates the source area or parent material is nearby, perhaps in a
nearby mountainous region. The source of the boulder should not be too far away because
if the boulder was transported by stream action its sharp corners and rough surface
probably would have been rapidly chipped away producing a much smoother, more
rounded texture. Figure 6 - 2 illustrates typical examples of rounded and angular clasts.
Very Angular
Sub - Angular
Sub - Rounded
Well - Rounded
Figure 6 - 2. Typically examples of rounded to angular clasts.
3. Sphericity
Sphericity is a measure of how spherical a clast is. If a clast has a shape that is very much
like a perfect sphere or ball then it is said to have a high sphericity. If a clast has an
elongated, flat or linear shape, then it is very much different in overall shape from a perfect
sphere and is said to have a low sphericity. Figure 6 - 3 illustrates typical examples of
mineral clasts with high, medium and low sphericity.
Like roundness, sphericity is also a good indicator of the processes a clast has gone
through, especially their intensity and duration. A clast with a low sphericity will likely
have spent only a short period of time in a lower energy environment. It will have been
eroded, transported and deposited in something like a braided river setting not too far
removed from its source area. On the other hand, a clast with a high sphericity has
probably spent more time in a more intense environment of erosion, transport and
deposition, such as a beach.
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GEOL 1107, Lab #6
Low Sphericity
Medium Sphericity
High Sphericity
Figure 6 - 3. Typical examples of mineral clasts with high, medium and
low sphericity.
4. Sorting
Sorting describes the range of clast sizes present in clastic sediment or a clastic sedimentary
rock. Like the other textural features, sorting is a very important characteristic because it
gives us clues as to the processes that are or have been at work.
Very Well Sorted
Well Sorted
Moderately Sorted
Poorly Sorted
Figure 6 - 4. Very well to poorly sorted clastic sedimentary rock textures
(Boggs, 1995, page 88).
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GEOL 1107, Lab #6
Very well sorted and well sorted clastic sediment is composed of clasts made up of
one or two dominant sizes as well as, typically, only one type of mineral or rock
composition (Figure 6 - 4). This type of clastic sediment reflects conditions of intense
and/or prolonged sediment transport during which the different clast size and
compositions have been effectively sorted into distinctly different deposits. Well
sorted sediment may accumulate in a large fast flowing river or in desert where
enough energy persists – moving water, moving air – to cause significant work to be
done to the sediment leaving it far from its original source. The difference between
very well sorted and well sorted sediment is the degree of intensity and duration of
the energy in the environment. Very well sorted sediment is often found in a beach
where wave, tides and currents never stop, and sediment is constantly being
reworked.
Poorly sorted clastic sediment is composed of grains of many different sizes, fine to
coarse, and that also have a mixed composition of mineral and rock types (Figure 64). This type of clastic sediment reflects conditions of weak, intermittent or short-term
sediment transport during which there is little time or energy for clast sorting. A
poorly sorted clastic sediment may be deposited by glacial ice, a braided river or by a
gravity flow. In all three depositional settings sediment transport is of short duration
and subjected to relatively weak energy of movement. They may typically occur close
to the sediment source area.
Moderately sorted clastic sediment is transitional between well and poorly sorted
sediment.
Compaction and Cementation
This is what happens to sediment after it has been deposited.
Compaction is a reduction in the size of a deposit of sediment, usually as a result of a
reduction in the void space between individual clasts, because of the weight of overlying
material. Compaction of sediment occurs as it is buried by and subjected to the weight of
overlying sediments. Pressure builds up inside the sediment and water and air are
squeezed out of the loose sediment. Mineral clasts inside a deposit that are subjected to
intense, long-term pressure may be bent or broken and then become sutured or joined
together to form a solid, cohesive rock. Compaction is especially prevalent in carbonate
sediments that are relatively soft and prone to distortion. Quartz sand, on the other hand,
is not as readily compacted because of the resistance of quartz minerals to changes due to
pressure.
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GEOL 1107, Lab #6
Cementation is the process by which mineral clasts or shell fragments are bound together
by minerals precipitated from water in the pore spaces between clasts. At the contacts
between clasts there may be partial dissolution of minerals because of pressure. Dissolved
minerals may migrate into adjacent void spaces between clasts where precipitation of a
“new” secondary mineral may take place. This cementation of clasts by a secondary
mineral helps to bind the sediments together to form a solid rock.
By the processes of compaction and cementation unconsolidated sediment is converted to
rock. This is known as lithification.
COMPOSITION
The vast majority of clasts in a clastic sedimentary rock are composed of only a few
different mineral types, all of which you’ve seen in previous labs. The most common
mineral types are:
Quartz
Plagioclase feldspar
Potassium feldspar
Muscovite
Biotite
Clay minerals such as kaolinite
Iron oxides such as magnetite and hematite
Other less common constituents of a clastic sedimentary rock may include lithic clasts
which are fragments or pieces of rock. Remember a rock is an aggregate of one or more
minerals. These may include clasts of basalt, sandstone, dacite, mudstone, syenite or
limestone.
Biogenic clasts may also constitute a significant part of a clastic sedimentary rock
(Figure 6 - 5). These are organic fragments such as calcite shell fragments and organic plant
remains.
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GEOL 1107, Lab #6
Figure 6 - 5. Biogenic clasts, in the form of shell fragments, make
up this entire fossiliferous limestone, a form of chemical sedimentary
rock.
Mafic minerals are relatively rare in sediment because these iron-bearing minerals are
more prone to weathering, especially oxidation, and disappear relatively fast from
sediment deposits depending upon conditions.
Maturity of a clastic sedimentary rock
Maturity is a term used to describe the sediment in a clastic rock and the work that has been
done to it. It also is used to describe the stability of the sediment. If a clastic sedimentary
rock is mature, it means that a significant amount of work has been done to it and that it is
more stable. A less mature clastic sedimentary rock is one that has had less work done to it
and is less stable. The amount of work that is done is reflected by the texture and
composition of the clasts in the rock. This work is done by moving water (i.e. river),
moving air (wind), moving ice (i.e. glacier) or gravity (i.e. avalanche). Stability is a measure
of how prone the sediment is to further change caused by more work. Specific factors that
determine the maturity of sediment and the clastic sedimentary rock that eventually forms
include:
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GEOL 1107, Lab #6
1. Type, duration and intensity of physical and chemical weathering affecting the
sediment
2. How the sediment was eroded or set in motion
3. Method, duration and intensity of sediment transport
4. Mode of deposition.
For example, a pile of sediment that has been transported many kilometers in a fast-flowing
river will be well sorted. Individual clasts will be small, well rounded, spherical and
probably composed of only a very few, resistant mineralogies such as quartz. If these
sediments are deposited on a riverbank during a flood and then exposed to intense
physical and chemical weathering, as might happen in a hot, humid climate, then they will
undergo even more alteration with the resulting loss of any remaining, immature or
unstable mineral clasts. The end result is a very mature or very stable sedimentary deposit
that is not likely to be further altered during any further reworking.
On the other hand, if a pile of sediment is not transported very far from its source, then the
clasts are likely to be on average large, poorly sorted, more angular, less spherical and
probably composed of a variety of different stable and unstable mineralogies. If this
sediment is deposited and rapidly buried by another depositional event, then it may be
effectively removed from the effects of further physical and chemical weathering. The end
product is an immature sediment that eventually becomes an immature sedimentary rock.
Therefore, the maturity of a sediment deposit or sedimentary rock is measured by its clast
size, sphericity, roundness, sorting and mineral composition. Maturity may also be broken
down into two types, chemical maturity and physical maturity.
Chemical maturity is determined by the mineralogy of sediment or sedimentary rock.
If a rock is chemically mature then it contains a majority of clasts that are composed of
stable minerals, such as quartz and kaolinite. Quartz is the most stable of all silicate
minerals. Ferromagnesian or mafic minerals, on the other hand, are the least stable
and normally weather quickly to a more stable secondary mineral such as an iron
oxide like hematite.
Physical maturity is determined by the texture of a sediment or the clasts in a
sedimentary rock. If a rock is physically mature, then it has undergone considerable
sorting and abrasion during transport and deposition. Individual clasts are smaller,
well rounded, spherical and the whole deposit is well sorted.
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GEOL 1107, Lab #6
The concept of maturity applies mostly to clastic sedimentary rocks as they are the only
class of sedimentary rock that routinely exhibits significant changes in texture and
composition. Breccias and conglomerates are considered immature. Mudstones, siltstones
and claystones are considered mature. Sandstones are found somewhere in between. The
concept of maturity is not normally applied to chemical sedimentary rocks such as
limestone and dolomite because they frequently form by other processes such as when
minerals precipitate or are produced biologically.
CHEMICAL SEDIMENTARY ROCKS
TEXTURE
The most common chemical sedimentary rocks are the carbonates, which include limestone
and dolomite. The texture of a chemical sedimentary rock is defined by the size and type of
the pieces that make up the rock, either clasts or crystals. The key to understanding the
texture of chemical sedimentary rocks is to know that clasts and crystals come from two
different sources. Clasts may have first been precipitated and then eroded, transported and
deposited to form a deposit. A very common source of these ‘reworked’ chemical
sedimentary rock clasts are calcium carbonate body parts of marine organisms or, what are
regularly referred to as shell fragments. When shell fragments are included in a chemical
sedimentary rock it is often described as having a ‘fossiliferous texture’. Crystals form
when minerals are precipitated, which often takes place in the same environment where
they are deposited. These crystals are ‘not reworked’. Chemical sedimentary rocks include
precipitates of halite or gypsum.
The inclusion of clasts or crystals in a chemical sedimentary rock results in two common
chemical sedimentary rock textures.
Fossiliferous Texture
Fossiliferous texture is a form of clastic texture in chemical sedimentary rocks. It is the
result of the accumulation of skeletal parts (i.e. shell fragments) of invertebrate life forms,
usually those found in marine environments (Figure 6 - 6). Hard, internal and external
calcium carbonate skeletal body parts are formed by many organisms which remove
calcium and carbonate from fresh or seawater and process it as part of their regular
metabolism. When these organisms die, these hard body parts remain and may be
concentrated as fragments in a depositional environment such as beach, lake, ocean bottom
or reef. As lithification takes place, the texture of the resulting rock takes on many of the
characteristics of the skeletal fragments. For example, a layer of mollusk shell remains that
becomes cemented together by secondary carbonate precipitates will become a solid,
cohesive carbonate rock with a fossiliferous texture. Similarly, a coral reef is nothing more
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GEOL 1107, Lab #6
than the remains of many carbonate secreting marine organisms preserved on the sea floor,
often in the positions in which they grew.
Fossil Shell
Fragment
Scale is in centimeters
Figure 6 - 6. Fossiliferous texture of a carbonate rock. This example contains
the partial remains is of shell fragments.
Crystalline Texture
Crystalline texture refers to those rocks composed of interlocking mineral crystals. These
crystals have grown in place as the newly forming mineral precipitated from atoms or
molecule dissolved in water. Limestone that precipitates in a hot desert lake environment
or in a cave in the form of a stalactite or a stalagmite has a crystalline texture. Halite or salt
precipitates to form a chemical sedimentary rock with a crystalline texture. The texture of a
crystalline chemical sedimentary rock is similar to an igneous rock.
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GEOL 1107, Lab #6
The average size of the crystals in a chemical sedimentary rock is a useful guide to further,
rock classification. Four class sizes are commonly used to help identify these rocks.
Coarse crystalline
> 2mm
Medium crystalline
0.0625 to 2 mm
Fine or microcrystalline
0.002 to 0.0625 mm
Cryptocrystalline
< 0.002 mm
Note that these are the same size classes used for clastic sedimentary rocks.
COMPOSITION
There are only five different types of mineral or organic material that make up the common
chemical sedimentary rocks. These are used to help classify and explain the mode of
formation of this type of rock.
1. Carbonates
Carbonate rocks are those that contain 50% or more carbonate minerals (calcite, dolomite
and travertine). These are the most abundant of all the chemical sedimentary rocks. They
are composed of newly formed mineral precipitates and the remains of fossilized biogenic
carbonate remains, so their formation is closely linked to the life cycles of marine and
freshwater organisms. Remember there are many aquatic organisms able to extract
dissolved calcium and carbonate from water to form protective shells or skeletons. When
these organisms die their hard body parts accumulate as sediment which later may be
cemented together to form a solid rock.
Carbonate rocks, therefore, are often rich in fossils which provide valuable information on
the type of environment where the organism lived and, after its death, how and where its
remains were deposited and preserved. For example, a carbonate reef encased in
microcrystalline limestone suggests that at some time in the past there was a warm, shallow
marine environment (necessary for reef formation) probably followed by a rise in sea-level
that resulted in deeper water conditions (necessary for microcrystalline limestone
formation).
2. Evaporites
Evaporites are chemical sedimentary rocks composed of minerals that precipitated out of
solution (i.e. water) because of either a drop in temperature or a reduction in the volume of
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GEOL 1107, Lab #6
solution. There is no biological process affecting precipitation. The most common
evaporite minerals are halite (NaCl), sylvite (KCl), gypsum (CaSO4 - 2H2O), and anhydrite
(CaSO4) which precipitate from water in hot, arid climates where temperatures are high and
evaporation of water exceeds input from rain or rivers. Evaporite formation is further
enhanced when water circulation is inhibited, such as in a closed lake basin, and the
concentration of dissolved minerals is allowed to increase.
3. Siliceous Rocks
Siliceous rocks are composed mainly of microcrystalline quartz (SiO2) which forms when
silica dissolved in water precipitates. Siliceous rocks include chert, flint, jasper, agate and
opal. Chert or microcrystalline quartz is the only one of these siliceous rocks we’ll see in
this course.
Siliceous chemical sedimentary rocks may be either inorganic or biochemical in origin. For
example, some cherts are precipitated from seawater under certain conditions, others are
derived from the accumulation of siliceous hard body parts produced by silica secreting,
unicellular organisms such as diatoms and radiolarians which are abundant in the world’s
oceans. These organisms, like many modern carbonate secreting organisms produce silica
as part of their normal metabolic activities.
4. Ironstone
Ironstones are a group of precipitated chemical sedimentary rocks that are rich in the ironbearing minerals hematite, magnetite and limonite (Figure 6 - 7). Ironstones form at the
Earth’s surface in unique depositional environments where precipitation can take place.
They were a common type of rock formation in the distant past (> 2 billion years ago) when
it is believed the Earth had a much different atmosphere (lower oxygen content) and life
forms were less common, much simpler and less diverse than they are now and iron
precipitation on a vast scale could take place.
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GEOL 1107, Lab #6
Scale is in centimeters
Figure 6 - 7. Ironstone, a chemical sedimentary rock
5. Organic Rocks
Organic rocks are not rocks in the strictest sense because they are composed mostly of
organic material and not minerals. These rocks form in certain special environments in
which either the rate of accumulation of organic material is very high, or the environment is
such that the rate of decomposition of organic matter is very low. Environments where
organic rocks form include swamps, marshes or stagnant lagoons where there is abundant
vegetation to supply organic material and water that’s low in dissolved oxygen so that
preservation potential is high.
Common organic rocks include peat (Figure 6 - 8) and coal (Figure 6 - 9) which are derived
from terrestrial plant remains. Peat is the precursor to coal which means that from a
deposit of terrestrial plants, first peat forms then coal. Peat forms when organic material is
buried and preserved near the Earth’s surface. Coal forms because of intense long-term
heat and pressure after peat is buried deeper inside the Earth.
Hydrocarbons such as oil and natural gas are formed from the partially preserved remains
of marine animals and plants. The most common source of hydrocarbons is the remains of
marine algae, a single-cell photosynthetic plant that thrives in the world’s oceans.
Hydrocarbons are not formed into distinct rock bodies. Normally the organic material that
forms oil and gas is deposited with very fine grain clastic sediment such as clay and silt that
eventually become a mudstone or shale. Black shales are a major source of oil and gas. The
black colour of these rocks is because of their high organic matter content.
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GEOL 1107, Lab #6
Scale is in centimeters
Figure 6 - 8. Peat, an organic sedimentary rock
Scale is in centimeters
Figure 6 - 9. Coal, an organic sedimentary rock
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GEOL 1107, Lab #6
SEDIMENTARY ROCK IDENTIFICATION
Sedimentary rock identification is based on the recognition of clastic, chemical and organic
types. Table 6 – 1 on page 26 summarizes all the common sedimentary rocks and their
characteristics.
CLASTIC SEDIMENTARY ROCK IDENTIFICATION
Clastic sedimentary rocks are identified according to their texture. When identifying a clastic
sedimentary rock, the first thing to do is recognize that the rock is made up of clasts or rock
fragments that are all stuck together. Once you see this kind of clastic or fragmental texture
you know you have a clastic sedimentary rock. The next step is determining the average or
most common clast size. There is no need to count clasts, a simple approximation will do.
Once you have the average clast size go to figure 6 - 1, page 4 to see what kind of clastic
sedimentary rocks have this average clast size. For example, if your rock sample is made up
mostly of clasts between 3 and 5 mm in size then you know the sediment is gravel and the
rock is either a breccia (Figure 6 - 10) or a conglomerate (Figure 6 - 11).
Scale is in centimeters
Figure 6 - 10. Breccia, a clastic sedimentary rock. The large clasts are angular.
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GEOL 1107, Lab #6
Scale is in centimeters
Figure 6 - 11. Conglomerate, a clastic sedimentary rock. The large clasts are rounded.
Now that clast size has been determined the next step is to look at other texture features
and the composition of the clasts. If the clasts are gravel size (> 2 mm) then the rock is
either a breccia or a conglomerate like the examples in the previous two photographs. The
difference between a breccias and a conglomerate is that in a breccia the gravel-size clasts
are angular while in a conglomerate the gravel-size clasts are rounded (Table 6 - 1).
What if the average clast size is 1 mm? Then the rock is a sandstone where clast size is
between 2 mm and 0.0625 mm. In Table 6 - 1 there are four different sandstones. Each
sandstone has a unique mineralogy that give it a distinct colour. For example, the rock in
figure 6 - 12 is a clastic sedimentary rock. We can see on close inspection that it’s made up
of compacted clasts (Figure 6 - 13). What’s more the clasts are all about 1 mm in size and
they are reddish pink in colour. Arkosic sandstone or arkose is commonly red, orange and
pink because of the high concentration of potassium feldspar clasts in this rock. Therefore
the rock in figure 6 - 12 and 6 - 13 is an arkosic sandstone. The colours and mineralogies of
the four sandstones are summarized in table 6 - 1.
What if the clasts in a rock sample are really hard to see? What if the average clast size is
less than 0.0625 mm? The clastic sedimentary rocks with silt and clay-size clasts are
mudstone, shale, siltstone and claystone. It’s difficult to distinguish a mudstone from a
siltstone from a claystone. For this course we will only ask you to identify a mudstone and
not a siltstone or a claystone. Here’s the reason for this. A mudstone is composed of both
silt and clay-size clasts. It’s possible to identify silt-size clasts (0.002 to 0.0625 mm in size)
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GEOL 1107, Lab #6
Scale is in centimeters
Figure 6 - 12. Arkosic sandstone, a clastic sedimentary rock.
Scale bar is 1cm
Figure 6 - 13. Close up of sand-size clasts in an arkosic sandstone.
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GEOL 1107, Lab #6
with the unaided eye, so it’s possible to identify a mudstone. It’s impossible to identify
clay-size clasts (< 0.002 mm) with the unaided eye, so it’s impossible to identify a claystone.
So, to ease identification we combine silt and clay-size clasts together call it mud and focus
on mudstone.
A shale is a layered mudstone (Figure 6 - 14). It may have internal layers, or a rock sample
may be shaped in such a way that it looks like it was once part of a layer. So, shale, along
with mudstone, are the two common fine grained clastic sedimentary rocks you need to be
able to identify for this course.
Scale is in centimeters
Figure 6 - 14. Shale, a layered clastic sedimentary rock
made up of fine, silt and clay-size clasts
To summarize, how do you identify a mudstone and a shale? Both are composed of silt and
clay-size clasts. A mudstone is not layered, it has instead a massive structure. A shale is
layered.
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GEOL 1107, Lab #6
CHEMICAL SEDIMENTARY ROCK IDENTIFICATION
Chemical sedimentary rock classification is a matter of mineral recognition. The dominant
mineral type in the rock dictates what type of rock it is. The rock in figure 6 - 15 is gray in
colour and reacts in solid form to 10% hydrochloric acid. It does not have to be ground up
into a powder for this reaction to take place. Therefore, this rock is made up of the mineral
calcite. Looking at the short list of common chemical sedimentary rocks in table 6 - 1 the
rock is a limestone.
There are only two types of organic sedimentary rock to look at in this lab, peat and coal,
and the difference between them is colour and density.
Scale is in centimeters
Figure 6 - 15. Limestone, a chemical sedimentary rock.
The rock in figure 6 -15 is dark gray, a colour not usually associated with the mineral
calcite. However, it is still a limestone because it reacts with HCl. Colour does not matter.
It maybe this limestone is dark gray because of impurities, such as organic material, that
were included when it formed.
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GEOL 1107, Lab #6
EXERCISE 6 - 1. THE FEATURES USED TO IDENTIFY A SEDIMENTARY ROCK
Examine the sedimentary rocks provided in the lab and answer the following questions.
1. Compare the texture of a coarse crystalline igneous rock, R28, to a clastic sedimentary
rock, R16c. Make sure you can recognize and understand the differences between these
fundamentally different rock types, especially their different textures.
2. Examine samples R66, R65a, R3a, and R16c. What do all four of these rocks have in
common? What is different about all four of these rocks? Note differences with respect
to mineralogy and size of grains or clasts, sorting, roundness and sphericity of grains.
Arrange them in order of relative maturity and assign names to each rock. Comment on
the relative maturity of each rock.
3. Examine samples R12, R40a, and 69. What fundamental aspect of classification do these
three samples have in common? Name each rock.
(Hint: Because you cannot see the individual mineral grains in each sample, you
probably cannot identify the constituents by sight alone. Therefore, you will have to test
for properties in order to identify the minerals in the rocks. Remember your scratch test,
acid test, streak test, etc).
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GEOL 1107, Lab #6
4. Name sample R17. Can you identify any of the rock types that make up the pebble-size
clasts? What kind of material forms the matrix between the pebble clasts?
5. Name sample R13. List 3 or 4 physical properties that distinguish this rock from the rock
obsidian. Aside from general appearance, what fundamental aspect do sample R13 and
obsidian have in common?
6. Name samples R13, #73, and R16a. Each of these samples is of very great economic
importance. What is each used for?
7. Examine sample R19. What are the clasts and matrix made of? Where might this rock
have formed?
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GEOL 1107, Lab #6
8. Examine and describe sample R76 (please do not scratch the material). Is this rock clastic
or chemical? What is the grain or crystal size? What is its relative maturity? Why is this
particular rock black in color? What does the presence of black material and the texture
of the rock suggest about its environment of deposition? Was this depositional setting
characterized by still or moving water?
EXERCISE 6 - 2: SEDIMENTARY ROCK IDENTIFICATION
Using table 6 - 1, pages 126 to 129, identify all the samples in the box of sedimentary rocks.
In this box there are clastic, chemical and organic rock types. Put your answers on the next
page in the appropriate blank spaces. Answers should include a description of mineralogy,
texture, structure presence or absence of fossil or other biogenic material.
24
R13
R66/R66a
GEOL 1107, Lab #6
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GEOL 1107, Lab #6
Table 6 - 1. Common Sedimentary Rocks
Part A. Clastic Sedimentary Rocks
Name
Average or
dominant
grain size
Color
Grains
or
Clasts
Coarse Grain Clastic Rocks (Avg. grain size > 2 mm)
Breccia
Coarse grain size,
greater than 2 mm
Varies. Usually
many different
colors.
Conglomerate
Coarse grain size,
greater than 2 mm
Varies. Usually
many different
colors.
Angular clasts.
Potentially many
different grain sizes,
including large and
small (pebbles,
cobbles and
boulders), in a finer
grain matrix
Rounded clasts.
Potentially many
different grain sizes,
including large and
small (pebbles,
cobbles and
boulders), in a finer
grain matrix.
Medium Grain Clastic Rocks – Sandstones (Avg. grain size 0.062 mm – 2
mm)
Quartz
Sandstone
Arkosic
Sandstone
Grain size between
0.062 and 2 mm.
White
> 90% quartz grains,
most often spherical
but difficult to see
due to small size.
Grain size between
0.062 and 2 mm.
Red, orange, light
pink to white
Abundant quartz and
at least 25%
K – feldspar
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Very
Mature
Mature
GEOL 1107, Lab #6
Name
Lithic
Sandstone
Graywacke
Average or
dominant
grain size
Color
Grains
or
Clasts
Grain size between
0.062 and 2 mm.
Usually medium
to dark gray,
brown or green.
Frequently varies
in color.
A mix of quartz,
feldspar and other
rock fragments. Rock
fragments are in
greater abundance
than feldspar. < 15%
silt and clay matrix.
Commonly between
0.062 and 2 mm,
with significant finer
grained matrix.
Usually dark gray,
may be black,
brown or green.
Frequently varies
in color.
A mix of quartz,
feldspar and other
rock fragments.
> 15% silt and clay
matrix.
Fine Grain Clastic Rocks (Avg. grain size < 0.062 mm)
Mudstone
Grain size less than
0.062 mm
Variable.
Frequently
medium to
dark gray or
black.
Sometimes
brown or
green.
Massive or nonlaminated structure.
Silt-size quartz, with
fewer feldspar
grains. Abundant
clay-size, clay
mineral grains.
Shale
Grain size less than
0.062 mm
Variable.
Frequently
medium to
dark gray or
black.
Sometimes
brown or
green.
Thin bedded or
laminated structure.
Silt-size quartz, with
fewer feldspar
grains. Abundant
clay-size, clay
mineral grains.
Different size
particles may be
concentrated in
different
laminations.
27
Immature
Very
Immature
GEOL 1107, Lab #6
Name
Average or
dominant
grain size
Siltstone
Grain size between
0.002 and 0.062 mm
Claystone
Grain size less than
0.002 mm
Color
Typically white
to light brown,
gray or red.
Sometimes
medium to
dark gray.
Commonly
medium to
dark gray or
black.
Frequently
brown or
green.
Grains
or
Clasts
Massive or, less
commonly with a
laminated structure.
Predominantly siltsize quartz, with
fewer feldspar
grains. Lesser
amounts of clay-size
mineral grains
Commonly has thin
bedded or laminated
structure.
Sometimes massive
in structure.
Abundant clay-size,
clay mineral grains.
Part B. Chemical and Organic Sedimentary Rocks
Name
Limestones,
including
fossiliferous
limestone
Dolomite
Color
Constituents, Components, Properties
Commonly white.
May also be red,
black, brown or gray
depending upon the
type of impurities.
Microcrystalline or coarse crystalline
calcite (CaCO3) matrix that may
contain recognizable fossil remains or
other carbonate fragments. Readily
reacts with 10% HCl. May exhibit
diagnostic features of the mineral
calcite.
Commonly white.
May also be red,
black, brown or gray
depending upon the
type of impurities.
Microcrystalline or coarse crystalline
dolomite (CaMg)(CO3)2. Does not
normally contain fossil fragments.
Does not react with 10% HCl unless
powdered. May exhibit diagnostic
features of the mineral dolomite.
28
GEOL 1107, Lab #6
Part B. Chemical and Organic Sedimentary Rocks (continued)
SEE THE MINERAL IDENTIFICATION TABLES 4 - 1 AND
4 - 2 FOR MORE DETAILS
Name
Chert
Ironstone
Gypsum
Salt or Halite
Peat and Coal
Color
Highly variable.
Individual samples
may exhibit a wide
variety of colors,
shades and
brilliance.
Usually red, black,
yellow or a mix of
all three colors.
White, gray,
yellowish, pink to
brown.
Frequently colorless,
but may be white,
pale yellow, red,
brown or black
depending upon
impurities.
Dark brown to black
Constituents, Components, Properties
Composed almost entirely of the
minerals micro-crystalline quartz
(SiO2) or opal (SiO2 . nH2O). Therefore
exhibits many of the diagnostic
features of these minerals, in particular
conchoidal fracture. May also have
banded or layered structure.
Composed of the iron-bearing
minerals hematite, magnetite, and/or
limonite, with less amounts of chert.
Exhibits features typical of these
minerals. Very hard. May be
magnetic. Also may have iron oxide or
“rusty” coating on outer surface due to
weathering and alteration.
Compact waxy-looking masses or
fibrous aggregates of elongated
crystals. Vitreous luster.
May appear as large individual
crystals, dense clusters of well formed
crystals or microcrystalline crusts or
coatings. Vitreous luster. Dissolves
readily in water. Distinctive “salty”
taste.
Composed of organic plant remains in
various stages of preservation. Peat
has a dull luster, low density and is
relatively easy to separate. Coal is
harder, more dense and, as is the case
with more altered “higher rank” coals,
may have a vitreous to glassy luster.
Both peat and coal may be banded or
layered with an uneven fracture.
29
GEOL 1107, Lab #7
LABORATORY 7
METAMORPHIC ROCK IDENTIFICATION
INTRODUCTION
Metamorphic rocks form when a pre-existing igneous or sedimentary rock is
subjected to heat and pressure. The result is a new type of rock that has
undergone some degree of solid-state alteration, which means new textures
and structures form from the older parent rock without significant melting.
While metamorphic rocks may be significantly different from their parent
material, they frequently retain features of the original igneous or
sedimentary rock. Recognition of new and inherited features is an important
consideration when it comes to metamorphic rock identification.
Heat and pressure producing metamorphic alteration is normally the result of
burial within the Earth, to depths on the order of several tens of kilometers, or
because of close proximity to active volcanic regions. Based on the relative
intensity of heat and pressure, there are three basic types of metamorphic
environment:
1. Regional metamorphism,
2. Contact metamorphism,
3. Dynamic metamorphism
Regional Metamorphism
Regional metamorphism is characterized by high heat and high pressure.
Typically, these types of conditions are found deep inside the Earth such as at
depth along an active tectonic plate margin where plates are pushing against
one another, and one plate is being subducted into the Earth’s hot interior.
Regional metamorphism results in intense, widespread alteration and
produces specific types of metamorphic rock, such as a schist or gneiss,
recognizable by their foliated or layered texture.
Contact Metamorphism
Contact metamorphism is characterized by increased heat with only slightly
elevated pressure. Contact metamorphism is found in areas of active
1
GEOL 1107, Lab #7
volcanism (lava is extruded onto or near the Earth’s surface) or plutonism
(magma is intruded into the Earth’s crust) where melted rock acts as a ready
source of heat. It’s also found along fractures or faults where the rock is in
contact with hot liquids or gases. In all these areas pressure is relatively low
because the depth beneath the surface is less and there is normally no large
scale tectonic plate motion. The intensity of contact metamorphism is
greatest at the contact between country rock and magma or lava, and
decreases rapidly with distance from the magma. Thus, zones of contact
metamorphism, known as “aureoles” or “baked zones”, are relatively
narrow. Contact metamorphism results in intense, but very localized
alteration producing specific types of non-foliated or non-layered
metamorphic rocks such as an amphibolite or a skarn.
Dynamic Metamorphism
Dynamic metamorphism is the result of increased pressure with only a minor
amount of extra heat. Dynamic metamorphism may be found along an active
plate margin such as a transform boundary where two plates are trying to get
past one another but there is no subduction and very little in the way of extra
heat flow. The dominant force is extra pressure or stress that acts to rearrange minerals such that distinctive new metamorphic layering or foliations
are formed.
Physical and chemical changes to a rock
Metamorphic rocks are altered without significant melting of parent material.
However, there is still important new mineral formation as original igneous
crystals and sedimentary clasts reform. Common trends associated with
changing mineralogy include the loss of some of the less stable minerals (i.e.
some types of feldspar) and the chemical recombination of old minerals
leading to the formation of new minerals (i.e. kyanite and sillimanite).
Typical textural changes include deformation and rotation of mineral grains,
the growth of larger crystals, often by the combination of crystals of the same
mineralogy, formation of layers or foliations that vary in thickness and
continuity, concentration of minerals in foliations, and in some instances the
loss of structure such as original sedimentary bedding or layers.
The net result of metamorphism upon a rock is larger crystals, increased
hardness, and new structural features that record the effects of heat, pressure
and stress in one or more directions.
2
GEOL 1107, Lab #7
TEXTURE
Texture in metamorphic rock is a measure of both clast or crystal size and
orientation. Metamorphic rocks are classified as either foliated (layered) or
non-foliated (non-layered).
FOLIATED TEXTURE
A foliated texture results from recrystallization and the growth of existing or
new minerals and their re-alignment in such a way that a layered rock is
produced. Normally a foliated texture is the result of stress. A rock is
subjected to unequal forces of stress such as tensional (pulling apart),
compressional (pushing together) or shearing (pushing past one another in a
side-by-side motion) which results in the preferential alignment of clasts or
crystals.
There are four types of foliated texture: slatey, phyllitic, schistose and
gneissic. These four textures form a continuum of products that reflect
increasing alteration due to more intense or prolonged heat and pressure.
3
GEOL 1107, Lab #7
1. Slatey texture is the lowest grade of foliated texture, it forms at the
lowest heat and pressure. It forms in fine-grained sedimentary rocks
with thin bedding planes (i.e. shale) that are less altered by
metamorphic processes. Under these so-called low grade
metamorphic conditions, minerals are only slightly re-aligned in a
preferred direction and the formation of new minerals is limited to a
relatively few, very fine-grained platy minerals such as muscovite
and chlorite. A rock with a slatey texture has the same very finegrained texture as the original shale or mudstone parent material
(Figure 7 - 1). A slatey texture is not that much different from the
original thinly bedded sedimentary structure of the shale parent
material. Its appearance is similar to bedding in the shale except that
it may be slightly easier to see.
Scale is in centimeters
Figure 7 - 1. The slatey texture of a foliated metamorphic rock
4
GEOL 1107, Lab #7
2. Phyllitic texture forms from a slatey texture as metamorphic
alteration continues. This texture is characterized by better defined,
more continuous foliations or layers of fine grained, occasionally
medium grained, platy minerals, the most common of which are
muscovite and biotite (Figure 7- 2 and 7 - 3). Foliations may be flat
and parallel or folded. The formation and concentration of platy
minerals along the flat surfaces of foliations and their preferred
orientation imparts a shiny to metallic luster to the rock surface.
Scale is in centimeters
Figure 7 - 2. The phyllitic texture of a foliated metamorphic rock
5
GEOL 1107, Lab #7
Scale is in centimeters
Figure 7 - 3. The phyllitic texture of a foliated metamorphic rock
6
GEOL 1107, Lab #7
3. Schistose texture forms next from a phyllitic texture as heat and
pressure increase and metamorphic alteration continues. Schistose
textures are characterized by moderately well defined, relatively thin
(< 5 mm), typically continuous to discontinuous foliations composed
of medium to coarse grained minerals, mainly consisting of platy
muscovite and biotite and more irregular or randomly shaped quartz
crystals (Figure 7 - 4). The texture of these rocks is characterized by
pronounced segregation of different mineral types. For example, in a
schistose rock you may see evidence of foliations containing a high
concentration of biotite (therefore black in colour) next to foliations
composed almost entirely of quartz or plagioclase feldspar (white in
colour). However, mineral segregation is usually just developing and
foliations of one mineral type are relatively thin and discontinuous.
A schistose texture is the product of an intermediate or higher grade
of metamorphism, meaning there was more heat and pressure for a
longer period of time.
Scale is in centimeters
Figure 7 - 4. The schistose texture of a foliated metamorphic rock
7
GEOL 1107, Lab #7
4. Gneissic texture forms from a schistose texture. A gneissic texture is
characterized by well defined, thick (> 5 mm, up to several
centimeters thick) continuous foliations, or in this case “bands”
composed of well segregated, coarse crystalline minerals. Bands
typically have well defined, “sharp” contacts or boundaries, an
important distinction between schistose and gneissic textures (Figure
7 - 5). The composition of a gneissic rock may be relatively simple, in
that it is a highly altered or metamorphosed rock in which only few,
more stable minerals (i.e. biotite, quartz, some feldspars) have
survived heat and pressure. Those mineral crystals that do survive
also tend to be large in size as they have grown at the expense of the
less stable varieties. Gneissic rocks may also be characterized by the
appearance of newly formed secondary crystals such as garnet,
staurolite, biotite and kyanite, up to several centimeters in size. The
growth of crystals in a gneiss may lead to a different texture where,
instead of thick foliations, there are large crystals in a massive
structure. In this case the crystals have grown so big that they are
thicker than the foliations (Figure 7 - 6).
Scale is in centimeters
Figure 7 - 5. The gneissic texture of a foliated metamorphic rock with well
defined foliations or layers.
8
GEOL 1107, Lab #7
Scale is in centimeters
Figure 7 - 6. The gneissic texture of a foliated metamorphic rock with a more
massive structure where the growth of large crystals has obscured the foliations
or layers
Lineations are another form of metamorphic structure. They are poorly
defined layers or foliation-like structures that form not because of new
mineral growth or mineral segregation but because pre-existing features in
the rock have been altered and preserved. For example, pebbles and sand
grains in a sedimentary rock were originally spherical. During metamorphic
alteration they may have been stretched by the stress acting on the rock. The
end result is a series of elongated grains, all aligned in one direction that
imparts a layered or lineated structure to the rock. Stretched pebbles are
common in some altered conglomerates or metaconglomerates. Deformation
of a limestone will often result in the preferred alignment of organic material
with the result that the altered rock now has lineations. While lineations are
not true foliations, like foliations, they give metamorphic rocks a layered or
foliated appearance.
9
GEOL 1107, Lab #7
NON-FOLIATED TEXTURE
A non-foliated texture develops when original minerals are recrystallized
and new minerals form in such a manner that no new layers can form and
pre-existing ones are obliterated. Growth of new minerals or the enlargement
of old ones occurs in all directions to form an interlocking crystalline
structure. For example, a quartz sandstone with well-defined bedding planes,
when subjected to heat and pressure normally loses its original sedimentary
structure as some minerals are lost and the more stable quartz grains are
compacted closer together. While non-foliated metamorphic rocks normally
do not contain parallel planes of minerals, they may contain stretched or
elongated fossils or other grains, like the lineations described in the previous
paragraph, that formed in response to metamorphic stresses with the result
that the rock has layered-like appearance.
OTHER TEXTURES
Porphyroblastic Texture
Some metamorphic rocks contain large crystals (porphyroblasts) set in a finer
grained groundmass (Figure 7 - 7). This porphyroblastic texture is similar to
the porphyritic texture of igneous extrusive rocks. It may be present in
foliated or non-foliated metamorphic rocks. Often the porphyroblasts or
large crystals are garnets or magnetite.
Garnet Crystals
Figure 7 - 7. Porphyroblastic texture in a metamorphic rock with large garnet crystals.
10
GEOL 1107, Lab #7
Granoblastic Texture
Metamorphic rocks that are equigranular, meaning they are made up of
mineral grains that are all one size, have a granoblastic texture. This texture
may be characteristic of both foliated and non-foliated rocks and includes
slates and phyllites (foliated rocks) as well as marbles and quartzites (nonfoliated rocks).
COMPOSITION
In general, metamorphic rocks are made up of some of the common minerals
also found in igneous and sedimentary rocks and a few minerals exclusive to
metamorphic rocks. The most common metamorphic minerals are:
quartz
plagioclase feldspar
potassium feldspar
muscovite
olivine
hornblende
biotite
galena
pyrite
chalcopyrite
calcite
dolomite
magnetite
chert
These minerals are also found in igneous and sedimentary rocks
11
GEOL 1107, Lab #7
Additional minerals exclusive to metamorphic rocks are:
chlorite
garnet
kyanite
staurolite
epidote
serpentine
METAMORPHIC ROCK IDENTIFICATION
The different types of common metamorphic rocks, with their features, are
summarized in Table 7 - 1 beginning on page 15. Metamorphic rocks are
divided into two categories: foliated and non-foliated. Once you’re familiar
with these types of rock the first step in metamorphic rock identification is to
determine if the rock is foliated or non-foliated.
IDENTIFYING FOLIATED METAMORPHIC ROCKS
The first question is “Does this rock have foliations or not?” If the rock does
have foliations, then you have four choices from which to select a name.
They are slate, phyllite, schist or gneiss. Based on what you see in a rock
needing identification and the images and descriptions for each rock type in
this lab, select the name that best fits the sample. Take for example the rock in
figure 7 - 9. It has foliations. There are numerous thick, continuous foliations
made up of coarse or large crystals with sharp, well defined contacts between
foliations. The rock in figure 7 - 9 is a gneiss. Note that there is no
requirement to identify the minerals in the rock. It’s useful to know what the
mineralogy of the rock is, and sometimes the names of the most common
mineral or minerals is included in the name, but it’s not necessary. For
example, the rock in figure 7 - 9 could be called a potassium feldspar gneiss
asthis is its most common mineral.
The rock in figure 7 - 10 is also a foliated metamorphic rock. It has very thin
foliations and none of the mineral crystals or clasts are visible. This rock also
has a polished sheen or luster. It is therefore a phyllite. If the flat surface of
the rock was dull, then it would be a slate.
12
GEOL 1107, Lab #7
Scale is in centimeters
Figure 7- 9. Potassium feldspar gneiss
Scale is in centimeters
Figure 7- 10. Phyllite
13
GEOL 1107, Lab #7
IMPORTANT NOTE
METAMORPHIC ROCK NAMES
Many metamorphic rocks may be given more specific names by pre-fixing
the textural rock name with the name or names of the dominant minerals
present in the rock. For example, a rock may have the texture and structure
of a gneiss, therefore it is a gneiss. If this rock is composed almost entirely
of quartz and biotite and it has numerous well defined garnet crystals,
then it may be called a quartz biotite garnet gneiss.
IDENTIFYING NON-FOLIATED METAMORPHIC ROCKS
There are only a few non-foliated metamorphic rocks to identify in this lab.
Their identification is based on the colour and mineralogy of the rock.
The first step in identifying a non-foliated metamorphic rock is to confirm
that it has no foliations or layers. Once that’s done the next step is colour.
What colour is the rock and therefore what minerals does it have. For
example, if a metamorphic rock is non-foliated and green then it’s a
greenstone. There is no need to identify the minerals in the rock, which are
often too small to see anyway. If the rock is white or light coloured with no
foliations, then it is either a quartzite or a marble. If this same rock reacts with
10% hydrochloric acid, then it’s a marble. The remaining few non-foliated
rocks are identified in the same way, comparing the colour and mineralogy of
the unknown sample to the list of rocks in table 7 -1.
14
GEOL 1107, Lab #7
Table 7 - 1. Common Metamorphic Rocks
Part A. Foliated Metamorphic Rocks
Name
Dominant Minerals
Texture and
Structure
Origins or
Parent material
Slate
Quartz, feldspar
minerals, muscovite,
biotite, chlorite,
Clay minerals.
Very fine grained
rock. Very thin,
difficult to observe
foliates. Dull
luster.
Mudstone, shale,
siltstone, claystone,
tuffs.
Quartz, feldspar
minerals, muscovite,
biotite, chlorite.
Fine grain mineral
crystals. Thin
foliations, easier to
observe than a
slate. Individual
mineral crystals
are usually too
small to observe.
However,
concentrations of
newly formed,
“larger”
muscovite and
biotite crystals
impart a
“polished” sheen
to foliation
surfaces.
Mudstone, shale,
siltstone, claystone,
tuffs.
Phyllite
15
Low grade
or little
alteration
Low to
medium
grade or
significant
alteration
GEOL 1107, Lab #7
Part A. Foliated Metamorphic Rocks (continued)
Commonly quartz,
muscovite, biotite,
fewer feldspar
minerals, chlorite
Schist
Gneiss
Commonly quartz,
muscovite, biotite,
fewer feldspar
minerals. Also
newly formed
metamorphic
minerals including
garnet, kyanite,
sillimanite, chlorite
andalusite, and
staurolite.
Fine to medium
coarse mineral
crystals. Thicker,
(up to 5 mm)
discontinuous
foliations that
impart a schistose
or foliated
structure.
Individual
foliations may be
easily observed
because of the
high concentration
of a particular
mineral (i.e.
quartz or biotite)
in each. Vitreous
luster.
Medium to coarse
mineral crystals.
Thick to very
thick, (up to
several tens of cm)
continuous
foliations impart a
gneissic structure.
Individual
foliations are
easily observed
because minerals
are now
segregated into
individual
foliations.
Vitreous luster.
16
Any Rock
Medium to
high grade
or
significant
alteration
Any Rock
High grade
alteration
GEOL 1107, Lab #7
Part B. Non-Foliated Metamorphic Rocks
Name
Greenstone
Dominant
Minerals
Texture and
Structure
Origins or
Parent material
Abundant chlorite
and epidote, and
lesser amounts of
actinolite,
plagioclase
feldspar,
calcite and maybe
quartz.
Typically green
because of chlorite,
epidote, and
actinolite.
Aphanitic or very
fine to fine grained
texture. Usually
massive or nonfoliated structure.
Mafic to intermediate
(rocks transitional
between mafic and
felsic) volcanic rocks
such as andesite and
basalt.
Quartz
Quartzite
Marble
Calcite
Dolomite
Fine to medium
grained texture.
Individual quartz
grains or crystals
may be easily
observed. Usually
massive texture.
Sometimes foliated
in appear as
original
sedimentary
structure survives
alteration.
Fine to medium
grained texture.
Individual calcite
grains or crystals
may be easily
observed. Usually
massive texture.
Sometimes foliated
in appear is
original
sedimentary or
biogenic structures
survive alteration.
17
Quartz Sandstone
Chert
Carbonate rocks
including limestone
and dolomite
GEOL 1107, Lab #7
Part B. Non-Foliated Metamorphic Rocks (continued)
Name
Skarn
Dominant
Minerals
Texture and
Structure
Origins or
Parent material
Calcite, pyrite,
chalcopyrite,
galena, sphalerite,
pyroxene, garnet,
oxide minerals.
Color highly
variable. Coarse to
very coarse
crystalline texture.
May exhibit a very
complex structure.
Massive,
heterogeneous
structure may also
include minerals
arranged in zones,
bands, nodules or
radiating masses.
Contact
metamorphism of
limestone and
dolomite, usually as a
result of magma
intrusion. Volatile
elements dissolved in
hydrothermal
solutions (hot water)
promote formation of
new, secondary
minerals.
Black or white and
black. Medium to
coarse crystalline
texture. Individual
crystals may or
may not be visible.
Usually massive
structure.
Mafic extrusive
(basalt lava, tuff) and
intrusive rocks
(gabbro) altered in a
regional
metamorphic
environment.
Dark green to
black. Medium to
coarse crystalline
texture. Massive
structure. Exhibits
many diagnostic
characteristics of
the mineral
serpentine.
Derived from the
alteration of
peridotite,
pyroxenite, and
dunite, and
sometimes
amphibolite and
gabbro, in the
presence of
hydrothermal
solutions (hot water).
Hornblende,
plagioclase feldspar
Amphibolite
Serpentinite
Serpentine,
magnetite, chlorite
18
GEOL 1107, Lab #7
EXERCISE 7 - 1: METAMORPHIC ROCK IDENTICATION
Examine the samples in the metamorphic rock display. Name each rock and
answer the questions below. Use table 7 - 1, “Common Metamorphic Rocks”
(pages 15 to 18) to help identify the different rocks.
1. What aspect of texture/structure do all of the first 4 samples in your box of
metamorphic rocks have in common?
2. Identify the mineral that forms the porphyroblast in sample R26a. What
type of rock did this metamorphic rock form from?
3. What is the pink mineral in the gneiss, sample R24?
4. What textural feature do samples R48, R47a, and R59 have in common?
Name each of these rocks and the original rock type it most likely formed
from.
5. Arrange rock samples R26a, R55, R70, R83, R25b, and R24 in order of
increasing metamorphism.
19
GEOL 1107, Lab #7
6. The samples in question 5 are the result of what kind of
metamorphism?
7. Examine the skarn sample, R82. What are the most common minerals in
this sample?
8. Using table 7- 1 (pages 15 to 18), name all the rocks in the metamorphic
rock box. Put your answers in the appropriate box in the metamorphic
rock identification answer sheet (page 21). Include the name of the rock,
the common minerals, and metamorphic texture.
20
GEOL 1107, Lab #7
R0
21