The building blocks of rocks
Seeds of growth
Modified from NIGS 2003 Geo11.1 Laboratory Manual
Introduction
The term “mineral” is used in many different ways. Jewelers recognize minerals as stones to be
cut, polished and mounted on different settings. Agriculturists enrich soils with fertilizers containing
different minerals to provide plants with nutrients to enhance their growth. Humans also need a certain
amount of minerals to supplement their everyday diet.
However, in geology, the word “mineral” has a specific connotation. In order for a substance to be
considered a mineral, it must meet five requirements. It must be a naturally occurring, inorganically
formed homogenous solid that has a definite chemical composition and an ordered internal
structure. These characteristics determine the mineral’s physical and chemical properties, which can be
used in their identification. In particular, six properties are commonly used in classifying minerals.
Color
Color is the composite of all visible light reflected back by the mineral. It is usually the most
noticeable physical property. For some minerals, color can be used as a diagnostic property, since these
minerals always occur in shades of one particular color (idiochromatic color). Examples are the intense
azure blue of azurite, the bright green of malachite, and the bright yellow of sulfur. However, most
minerals, like quartz, occur in a range of colors, due to impurities caused by chemical substitution and the
effects of radiation (allochromatic color). Some minerals have a characteristic iridescence (a lustrous
rainbowlike play of color), or have one color on freshly broken surfaces and a different color on tarnished
surfaces.
The color of a mineral depends on its absorption or reflection of certain wavelengths of light when
light is transmitted through or reflected on it. A white mineral appears white because it essentially reflects
the entire visible spectrum. A transparent mineral transmits the entire visible light spectrum. A black
mineral absorbs all wavelengths of visible light. Colored minerals selectively absorb certain wavelengths
of light and transmit or reflect the remaining light that our eyes see.
Luster
Luster is a description of how the surface of a substance reflects light. The nature of the reflecting
surface gives rise to the different kinds of luster, and the amount of reflected light produces varying
intensities of luster.
• Metallic – opaque and reflective, like the silver, gold, or copper sheen displayed by metals
• Submetallic -- intermediate between metallic and nonmetallic; describes materials that are
opaque or nearly opaque but reflect light well
• Adamantine -- the luster of diamond; displays extraordinary brilliance and shine
• Vitreous -- reflects light like glass; exhibited by 70% of all minerals
• Resinous – similar to the way amber or clear candy would reflect light
• Satiny – reflects light like satin cloth; exhibited by minerals with fibrous habits
• Pearly – similar to the luster exhibited by pearls or the inside of mollusk shells
• Waxy – reflects light like a wax candle
• Greasy – reflects light as if coated with a layer of grease
• Dull – nonreflective, like concrete or dirt; usually exhibited by rough, porous surfaces
A comparison chart is a simple tool that can be made to compare different kinds of luster properly. Take a
piece of folder or illustration board and cut it to a strip of 1.5 in x 6 in. Draw six or seven equal sized
squares as shown in Figure 2-1. On the squares, glue or rub materials that have luster types similar to the
metal foil
(metallic)
pencil rub
(submetallic)
)
oil pastel rub
(dull)
candle drip
(waxy)
Figure 2-1 Luster comparison chart
silk piece
(silky)
window glass
(vitreous)
ones mentioned in the list. You will then be able to compare these materials with the mineral specimens
in the exercise.
Streak
Streak is the color of a substance in its powdered form. It is often called a mineral’s true color,
since the streak of a mineral is constant regardless of its body color or clarity. Streak is thus a more
diagnostic property than color. Some minerals have streak colors that are similar to their body colors, like
those of azurite and malachite. However, most metallic minerals show streaks different from their body
color. For example, both hematite and magnetite can have body colors of black or very dark gray. But
hematite always shows a red-brown streak similar to the color of dried blood. Magnetite always has a
black streak.
The usual method for determining this property is by scratching the mineral against a piece of
unglazed porcelain tile called a streak plate, as long as the mineral is less hard than the porcelain plate
(see Hardness). The color of the powder left on the plate is the streak of the mineral. Other methods of
determining streak are by powdering a piece of the mineral with a hammer, or by scraping the mineral
with a pocket knife.
Hardness
Hardness is a measure of the resistance of a mineral to scratching and abrasion, and is usually
measured in comparison to other minerals of known hardness. A harder substance will scratch a softer
one. In 1812, a German mineralogist named
Friedrich Mohs developed a quantitative scale of
relative mineral hardness on which the softest
Table 2-1 Mohs Scale of Hardness
mineral (talc) has a hardness of 1, and the hardest
Common Objects
Hardness
Mineral
mineral (diamond) has a hardness of 10 (see Table
2-1).
1
Talc
There are a few points to remember when
2
Gypsum
2.5 Fingernail
testing for hardness:
3
Calcite
3.5 Copper wire
1) Always test a smooth, fresh surface of the
4
Fluorite
mineral. Weathered or corroded surfaces
4.5 Iron nail
will be softer than normal.
5
Apatite
5.5 Pocket knife
2) Cross-check the results of your hardness
6
Orthoclase
6.5 Streak plate
tests. For example, in addition to using
7
Quartz
mineral B to scratch mineral A, also try to
8
Topaz
using mineral A to scratch mineral B.
Without this cross check you may not be
9
Corundum
able to tell which minerals is actually doing
10
Diamond
the scratching.
Cleavage & Fracture
Minerals break along flat surfaces, irregular surfaces, or a combination of both. The flat breaks
are called cleavage planes, and the irregular breaks are called fracture surfaces.
Cleavage planes are sets of parallel surfaces representing weak chemical bonding between
repeating, parallel layers of atoms in a crystal. Each group of parallel cleavage planes has an orientation
relative to the crystal structure and is referred to as a cleavage direction. Different minerals have different
numbers of cleavage directions, depending on their inherent crystal structures. Minerals like muscovite
and biotite have basal cleavage, breaking in one direction into thin sheets like pages of a book. Halite has
three cleavage directions developed at right angles to one another, so it breaks either into cubes (cubic
cleavage) or fragments that have sides at right angles to each other.
Cleavage can be described as excellent, good, poor, or absent, depending on how well the
surface reflects light. When cleavage is absent—that is, when the mineral breaks very poorly and
irregularly—then the surface of the break is called a fracture. Minerals like quartz have characteristic
curved fracture surfaces that are called conchoidal fractures, much like that observed in broken glass.
Other Properties
Habit denotes the general shape of crystals. All true minerals have a crystalline structure, or an orderly
three-dimensional arrangement of atoms or molecules, which is manifested externally by the different
geometrical shapes that mineral crystals occur in. Pyrite is commonly found as cubes or intergrown
cubes. Muscovite and biotite generally grow as book-like stacks of thin sheets.
Specific gravity is the ratio of the weight of a substance to the weight of an equal volume of water.
Minerals made up of elements of high atomic weight are denser than minerals made up of elements with
low atomic weights. Minerals whose atoms are strongly bonded to each other have closely packed
structures, and are denser than minerals having similar chemical compositions but weakly bonded
components. The average specific gravity of minerals is about 2.7, but some metallic minerals have a
specific gravity many times greater than the average, depending on their purity. Platinum has a specific
gravity of about 21.4, while quartz has a specific gravity of 2.65.
Magnetism is a characteristic of minerals, such as magnetite, that have a high iron content and are
attracted to a magnetic field.
Reaction to acid differs among minerals. A drop of dilute hydrochloric acid applied on the fresh surface of
a carbonate mineral (a mineral whose chemical composition includes carbonate, CO 3) like calcite will
effervesce or “fizz” due to the release of carbon dioxide gas.
Over 2,000 minerals have already been discovered and named according to their characteristic chemical
composition and physical properties. Of these, only 20 are common and fewer than 10 form the bulk
composition of most rocks. Each of these minerals possesses unique properties that serve to identify
them from the rest.
Figure 2-2 Comparison of cleavage surfaces
Excellent cleavage (large even flat surfaces)
light rays
Good cleavage (small uneven flat surfaces)
Poor cleavage (a few small flat surfaces)
Fracture surface (no flat surfaces), or absent
cleavage
SOURCES
David, C.P., R.A. Amores, J.A.L. Barretto, R.R.C. Luis, E.J. Marquez, and K.L. Queaño. Geology 11.1
Manual of Exercises. Quezon City: UP Press, 1996.
Electrical and Magnetic Properties. <http://www.tmm.utexas.edu/npl/mineralogy/Science_of_Minerals/
electric_magnet_properties.htm>.
Hurlbut, C. Jr., and C. Klein. Manual of Mineralogy (after James D. Dana). John Wiley & Sons, 1977.
Mechanical Properties. <http://www.tmm.utexas.edu/npl/mineralogy/Science_of_Minerals/
mechanical_properties.htm>.
Mineral Properties: Luster. <http://www.minerals.net/resource/property/luster.htm>.
Optical Properties. <http://www.tmm.utexas.edu/npl/mineralogy/Science_of_Minerals/
optical_properties.htm>.
Tarbuck, E.J., F.K. Lutgens, and K.G. Pinzke. Applications & Investigations in Earth Science. New Jersey,
USA: Prentice Hall, 2000.
WORLD WIDE WEB LINKS
The Science of Minerals. <http://www.tmm.utexas.edu/npl/mineralogy/Science_of_Minerals/index.htm>
Exercise 4
The building blocks of rocks
Seeds of growth
Modified from NIGS 2003 Geo11.1 Laboratory Manual
Objectives
One of the most basic skills that a student of Geology needs to have is the ability to identify
minerals, since they are essentially the building blocks of the different kinds of rocks. To be able to
understand the origin, classification, and subsequent alteration of rocks, the student must be able to
recognize the minerals that make them up.
This exercise is designed to enable students to:
• become familiar with the physical properties of minerals.
• systematically identify minerals based on their physical properties.
Materials
To be provided by the students: pencil, eraser, pocket knife or glass plate, magnet, hand lens (10x)
To be provided by the teacher: mineral specimens, streak plates, dilute HCl
Procedure
To properly identify a mineral, one must first determine as many of its physical properties as
possible, employing the available tools. Remember that the goal of this exercise is for the student to
learn the systematic procedure of identifying minerals through observation and not simply to name them
based on gut feel or good luck.
List down as many identifiable physical properties as possible for each mineral in Table 2-3.
Additional observations are encouraged. Then, using the Mineral Identification Key (Table 2-2), narrow
down the choices and arrive at a mineral name.
Answer concisely the questions preceding Table 2-3 regarding minerals.
Light colored
Dark colored
Dark colored
Table 2-2. Mineral Identification Key
Adapted from Tarbuck, Lutgens & Pinzke (2000)
H=hardness, SP=specific gravity
Metallic Minerals
Name
Hardness
Streak
Other Diagnostic Properties
(Chemical
Composition)
Black; magnetic; H=6; SP=5.2; often
Magnetite,
Black
granular
Fe3O4
Brass yellow; H = 6; SP=5.2;
GreenishPyrite,
Harder than pocket knife
generally an aggregate of cubic
black
FeS2
crystals
Gray or reddish brown; H=5-6; SP=5;
Hematite,
Red-brown
platy appearance
Fe2O3
Greenish- Golden yellow; H=4; SP=4.2;
Chalcopyrite,
Softer than pocket knife
black
massive
CuFeS2
but harder than a
Silvery gray; H=2.5; SP=7.6 (very
Galena,
fingernail
Gray-black
heavy); good cubic cleavage
PbS
Silvery gray; H=1 (very soft); SP=2.2;
Graphite,
Softer than a fingernail
Dark gray
massive
C
Nonmetallic Minerals
Name
Hardness
Cleavage
Other Diagnostic Properties
(Chemical
Composition)
Black to greenish black; H=5-6;
Augite (Pyroxene),
SP=3.4; fair cleavage, 2 directions at (Ca,Na)(Mg,Fe,Al,Ti)(Si,
Al)2O6
nearly 90
Present
Hornblende
Black to greenish black; H=5-6;
(Amphibole),
SP=3.2; fair cleavage, 2 directions at
Ca2(Mg,Fe)4Al(Si7Al)O22
Harder than
nearly 60 and 120
(OH,F)2
pocket knife
Gray to brown; H=9; SP=4;
Corundum,
hexagonal crystals common
Al2O3
Dark brown to black; H=7;
Smoky quartz,
Not prominent
conchoidal fracture; glassy luster
SiO2
Olive green; H=6.5-7; small glassy
Olivine,
grains
(Mg,Fe)2SiO4
Dark brown to black; H=2.5-3;
Biotite,
Present
excellent cleavage in 1 direction;
K, Mg, Fe, OH, Al
elastic in thin sheets; black mica
silicate
Softer than pocket
knife but harder
Bright green; H=3.5-4; green streak;
Malachite,
than a fingernail
effervesces in HCl;
Cu2CO3(OH)2
Deep azure blue; H=3.5-4; pale blue
Azurite,
Not prominent
streak;
Cu3(CO3)2(OH)2
Softer than a
Reddish brown; H=1-5; SP=4-5; red
Hematite,
fingernail
streak; earthy appearance
Fe2O3
Potassium feldspar,
Pink to white
H=6; SP=2.6; 2
KAlSi3O8
directions of
Present
Plagioclase feldspar,
cleavage at
White to gray
NaAlSi3O8 to
Harder than
nearly 90
CaAl2Si2O8
pocket knife
Any color; H=7; SP=2.65; conchoidal
fracture; glassy appearance;
Quartz,
Present
varieties: milky, rose, smoky,
SiO2
amethyst, citrine
Nonmetallic Minerals (continuation)
Light colored
Hardness
Cleavage
Other Diagnostic Properties
White, yellowish to colorless; H=3; 3
directions of cleavage at 75
(rhombohedral); effervesces in HCl;
often transparent
Softer than pocket
White to colorless; H=2.5; 3
knife but harder
Present
directions of cleavage at 90 (cubic);
than a fingernail
salty taste
Yellow, purple, green, colorless;
H=4; white streak; translucent to
transparent; 4 directions of cleavage
Colorless; H=2-2.5; transparent and
elastic in thin sheets; excellent
cleavage in one direction; lightcolored mica
White to transparent; H=2; when in
Present
sheets, is flexible but not elastic;
varieties: selenite (transparent, 3
directions of cleavage), satin spar
(fibrous, silky luster), alabaster
(aggregate of small crystals)
Softer than a
White, pink, green; H=1-2; forms in
fingernail
thin sheets; soapy feel; pearly luster
Yellow; H=1-2.5; smells like rotten
eggs
White; H=2; smooth feel; earthy
Not prominent odor; when moistened, has a typical
clay texture
Pale to dark reddish brown; H=1-3;
dull luster; earthy; often contains
spheroidal-shaped particles; not a
true mineral
Name
(Chemical
Composition)
Calcite,
CaCO3
Halite,
NaCl
Fluorite,
CaF2
Muscovite,
KAl2(Si3Al)O10(OH,F)
Gypsum,
CaSO4·2H2O
Talc,
Mg3Si4O10(OH)2
Sulfur,
S
Kaolinite,
Al2Si2O5(OH)4
Bauxite,
FeO(OH) + Al2O32H2
Name
________________________________
Student Number
____________
Section
__________
Answer Sheet
The building blocks of rocks
Questions
1. What determines the hardness of a mineral?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
2. Why do some minerals cleave and others fracture?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
3. List five minerals and give their economic uses.
Mineral name
Economic use
a.
b.
c.
d.
e.
References
_____________________________________________________________
_____________________________________________________________
Specimen
Number
Luster
Hardness
Color
Streak
Fracture/Cleavage (# dirs.,
angle of intersection
Other Properties
Mineral Name