Geology 115/History 150
Name(s):
Lab 6: Minerals and metamorphic rocks
Minerals
A mineral is a naturally-occurring, solid, usually inorganic element or
compound with a definite crystal structure and chemical composition which
varies only within specific limits. Rocks are merely aggregates of minerals.
The mineralogical composition of a rock depends on the conditions under
which that rock formed. Igneous rocks tend to have minerals that form at high
temperatures; sedimentary rocks contain minerals that are stable at Earthsurface conditions. Metamorphic rocks consist of minerals that form under a
range of pressure and temperature conditions within the Earth.
Common rock-forming minerals:
Minerals in
igneous rocks
Minerals in
metamorphic rocks
Minerals in
sedimentary rocks
Quartz
Orthoclase
Plagioclase
Biotite
Muscovite
Amphibole
Pyroxene
Olivine
Quartz
Biotite
Muscovite
Amphibole
Garnet
Talc
Chlorite
Staurolite
Kyanite
Orthoclase
Plagioclase
Quartz
Orthoclase
Biotite
Muscovite
Calcite
Halite
Gypsum
Clay minerals
Mineral identification
The first part of this lab is to identify mineral specimens, using the charts
provided in the Geology Lab Manual. Note that most rock samples will not have
minerals as large as the ones you will see in this part of the lab, so take notice of
diagnostic characteristics that do not depend on mineral size.
1. For instance, consider color, a seeming obvious choice: find the quartz display
in the cabinet in the back of the lab room. Quartz is a mineral that also happens
to be a gem in some of its forms (e.g., opal, tiger’s eye, amethyst). Consider all
the different varieties of quartz; is there are unique color for quartz?
2. Now consider shape, another fairly obvious characteristic. Again examine the
quartz display in the cabinet. Is the shape uniform for both specimens of the same
mineral? In fact, if a mineral is left undisturbed as it precipitates, it can develop its
crystal growth habit. The shape of such minerals is generally attractive, or, as
mineralogists call it, “euhedral”.
Needed: Mineral testing kit (located in the Tub 1 space) and mineral samples M1 through M-14 (Tub 2). Please label the minerals with their M-numbers (use the
lab tape and a pen) so that they can be returned to their rightful box.
Using the charts:
So what characteristics are actually useful in identification? It turns out that the
chemical composition of a mineral (which distinguishes one mineral from
another, usually) manifests itself in certain ways.
The most apparent of these is the mineral's luster, which can be metallic or
non-metallic. Luster refers to how the mineral reflects light; a metallic luster is
how a piece of steel or bronze or copper would reflect light. Compare a piece of
metal's luster to the luster of a piece of glass; the glass' luster (vitreous) is not a
metallic luster. Of course, if the mineral has a dull or pearly luster, it is a nonmetallic luster.
3. a. Look at minerals M-1, M-3 and M-7. Only one of these samples has a
metallic luster. Which one?
b. Now examine minerals M-2 and M-10; again, only one of these has a metallic
luster. Which one? Hint: you may need to look at different specimens of the same
mineral. Why was this question harder to answer than part a?
Next, recall how color was problematic. However, we can still use it appropriately
in mineral identification by simply determining if the mineral is dark-colored
(black or one of the “cool” colors) or light-colored (white or one of the “warm”
colors). The difference is due to the particular chemical elements that make up
the mineral.
4. Look at samples M-11 and M-13. Which one is dark-colored? Which is light?
Now use the mineral identification charts found in the colored pages in the lab
manuals located in the tub labeled “Lab Manuals”. There are three sheets: yellow,
red and blue and they are divided according to the two useful characteristics
already mentioned.
Since you’ll be eventually filling out the table on the next to last page of this lab,
you can now enter the two useful characteristics for all of the minerals in this lab
(M-1 through M-14) into the table: luster and color.
To identify a mineral’s name, once you have determined a mineral’s luster and
color, note that you will be using one of the three colored sheets. Follow the
headings of the sheet left to right on the sheet to determine what test to do next.
For all minerals, the relative hardness of the mineral may be determined by
scratching a corner of the mineral on a piece of glass (or scratching a corner of
the glass plate on the mineral). Hardness is the mineral's ability to resist
scratching or abrasion. A mineral will scratch all softer minerals and will be
scratched by all harder minerals. Certain index minerals define the Mohs
Hardness Scale, so you can get a numerical value for hardness. Rather than
finding the exact numerical value, you will simply need to determine whether the
mineral is harder than glass.
5. Use the corner of a glass plate and scratch minerals M-1, M-6 and M-10 and
record the results below. Then scratch a corner of each mineral on the flat surface
and record the results. Be sure to brush off any flakes of mineral to make sure
that you’ve actually left a scratch! Then combine the information to draw a
conclusion. Hint: there’s one of each “type”.
Mineral
Does the glass
scratch the
mineral?
Does the
mineral scratch
the glass?
The mineral is “harder
than”, “softer than” or
“the same hardness as”
the glass?
M-1
M-8
M-10
Again for all minerals. cleavage is another property that helps narrow down the
identity of the mineral. Cleavage is the ability of the mineral to split along closely
spaced parallel planes. The planes along which a mineral cleaves (when hit with a
hammer, for instance) are the planes where all the weak atomic bonds in the
crystal structure exist. Notice that if all bonds are uniformly strong (like in a piece
of quartz), the mineral will not cleave along a plane; instead, it will break
unevenly and roughly...it will fracture. Cleavage is sometimes confusing because
some minerals have good cleavage, some have poor cleavage and still others have
no cleavage (they fracture). The table below should help identify different types of
cleavages, but ask if this concept is confusing!
Also note that we discounted shape a while ago. Cleavage is not the same thing as
shape – shape is how the mineral crystal grew, whereas cleavage is how the
crystal broke.
6. a. How many cleavages does M-7 have? Hint: It’s called a “sheet silicate” for a
good reason!
b. How many cleavages does M-10 have? Remember not to count parallel faces
twice. What angle separates each distinct cleavage?
c. Look at the display of the quartz spar crystal. Note that the top of the crystal
has a nearly perfect six-sided symmetry; then examine the bottom of the crystal
where it was broken off. How many cleavages does this chunk of quartz have? So
what cleavage-related property does quartz have?
Then there are more specialized tests that are not applicable to all minerals, but
for the minerals that the test works on, they are diagnostic!
For instance, if the mineral has a metallic luster, determine the mineral's streak
color. Streak refers to the color of the powderized mineral, most easily
accomplished by rubbing a corner of the mineral sample against the porcelain
streak plate provided.
7. a. Use the porcelain streak plate on sample M-4; what color does the streak
turn out to be? Is it the same as the color of the mineral?
b. Try streaking a few of the nonmetallic luster minerals. What seems to be the
problem with the streak test and nonmetallic minerals?
For minerals with metallic luster, magnetism may be used to identify the
mineral.
8. Use the magnet to determine if M-2, M-3 or M-4 is magnetic. “Weakly
magnetic”, “strongly magnetic” and “not magnetic” are acceptable answers. In
addition to writing your answer here, enter the information under “other
properties” on the appropriate row of the table.
9. Another test is a mineral’s reaction to weak acid. Obtain an acid dropper
bottle and place one drop of (hydrochloric) acid on a sample of M-1 and M-8.
Which one reacts? How can you tell?
10. All right, now put it all together and identify the rest of the mineral samples.
To make this simpler, here are the mineral names in alphabetic order:
amphibole, biotite, calcite, chalcopyrite, galena, hematite, magnetite, muscovite,
orthoclase (potassium feldspar or K-spar), plagioclase feldspar, pyroxene, pyrite,
quartz.
Mineral ID Chart
Other
Mineral Name
Sample Luster Streak Hardness Cleavage/
(if it is
useful)
M1
M2
M3
M4
M7
M8
M9
M10
M11
M13
M14
M17
M41
M42
M43
(relative
to glass)
Fracture
Properties
11. Mineral summary: Name three physical characteristics you can use to
distinguish quartz from calcite.
Mining economics
Consider the following selected assay results for layers of the Homestake breccia
deposits, as reported by Noranda Exploration and Crown Butte Mines (Van
Gosen, 2005).
Layer number
1
2
3
4
5
6
7
8
9
Thickness
(meters)
37.3
15.1
75.3
57.4
36.6
29.1
21.3
22.9
25.9
Gold tenor
(opt)
0.22
0.49
0.32
0.24
0.51
0.24
0.44
0.27
0.24
Silver tenor
(opt)
0.48
1.10
1.31
0.59
1.15
1.93
0
2.67
2.01
Copper tenor
(opt)
0.57
1.01
1.27
0.90
0.43
1.75
0.66
0.98
0.82
12. Given that the tenor (concentration) of the various metals in each layer is
report in troy ounces per ton of rock mined, for layer 1, how many tons of rock
would have to be mined to yield 1 troy ounce of gold? The fixed costs of
excavation, hauling, crushing and refining run about $480/ton of rock
mined; is this layer economically feasible to mine for gold? Why or why not?
13. As of May 9, 2016, gold’s spot price in New York is $1266/troy ounce, silver
is $17/troy ounce and copper is $0.13/troy ounce. Per ton of rock mined,
which layer number is the most economic to mine? Prove it with a calculation. Is
the silver and copper ever economic to mine here?
Metamorphic rocks
Metamorphic rocks have been subjected to sufficient heat and/or pressure to
melt some of their constituent minerals, but not all of them. As a result of this
selective mobilization of chemicals, only certain chemical reactions can occur,
and so a whole new set of metamorphic minerals are crystallized.
Throw in the presence of fluids such as water and carbon dioxide (yes, at these
pressures, even carbon dioxide can be a liquid), and nature has the means to
create even more metamorphic minerals and therefore metamorphic rocks. Note
that metamorphic rocks must be formed at depth; metamorphism is not a surface
process, and so is distinguishable from mere sedimentation.
Rocks that have foliation (a sort of wavy layering, though it can resemble
horizontal layering) are metamorphic rocks; the foliation indicates that
directional pressure was applied to the rock while the mineralogical changes
were occurring. On the other hand, some metamorphic rocks are not foliated;
they appear crystalline, like coarse-grained igneous rocks. These metamorphic
rocks were subjected to isotropic, or nondirected, pressure.
Because there are so many metamorphic minerals (of which you have seen but a
few), there are all sorts of ways to name metamorphic rocks. We will concentrate
on naming rocks by their metamorphic grade (that is, by the maximum degree
of heat and pressure they were subjected to, and not their mineral composition),
or, in some unusual cases, by their apparent composition (for instance, rocks
like marble, quartzite or metaconglomerate, from which you cannot
determine the metamorphic grade).
The parent rock of a metamorphic rock is the original rock that was
metamorphosed into what you see today. As you can see from Table 6.1, the
parent rock’s minerals really do determine the resulting metamorphic rock’s
composition. Note the differences in mineralogy even at the same grade.
Table 6.1— Mineralogy of metamorphic rocks related to parent rock and grade
MetamorFacies
phic
grade
Low
Zeolite
Greenschist
Medium
Amphibolite
High
Granulite
Basalt
Parent rock
Shale
Calcite, chlorite, zeolite Zeolite, sodium-rich
micas
Chlorite, amphibole,
Chlorite, muscovite,
plagioclase, epidote
plagioclase, quartz
Amphibole, garnet,
Garnet, biotite,
plagioclase, quartz
muscovite, quartz
Pyroxene, plagioclase, Biotite, orthoclase,
garnet
quartz, andalusite
A metamorphic facies is a name of a set of metamorphic minerals which is
uniquely created at a particular pressure and temperature. So, in addition to a
metamorphic grade, a rock can belong to a particular metamorphic facies as well!
Confused? You bet! However, realize that these terms all have their uses.
Note that not all minerals in a given cell in the table above will show up in every
specimen of that grade/facies/parent rock, but all minerals in the specimen will
be named in the cell!
One other consideration: there are three different types of metamorphism,
related to the particular tectonic setting of the metamorphism. As you are aware,
the deeper rocks are drawn into the lithosphere, the higher the temperatures and
pressures the rocks are subjected to. This is called regional metamorphism.
However, there are two other sets of conditions.
Blueschist-type metamorphism occurs under high-pressure but lowtemperature (high P, low T) conditions. Contact metamorphism occurs under
high-temperature but low-pressure (high T, low P) conditions. This means that,
depending on the tectonic setting, three different metamorphic rocks could arise
from the same parent rock. Table 6.2 summarizes these types.
Table 6.2 — Mineralogy of metamorphic rocks related to parent rock and grade
Meta.
type
Regional
Dynamic
(low grade)
Dynamic
(high grade)
Contact
Facies
Parent rock
Shale
See table 6.1
Blue amphibole,
Blue amphibole,
chlorite, Ca-silicates
chlorite, quartz
Pyroxene, garnet,
not observed
kyanite
Pyroxene, plagioclase
Andalusite, biotite,
orthoclase, quartz
Basalt
Blueschist
Eclogite
Hornfels
Needed: Samples M18 and M 19 (Tub 37), R34 through 45 (Tubs 38 – 49)
14. Some minerals are made under metamorphic conditions. You have seen some
of them previously in this lab. Two other metamorphic minerals are kyanite and
chlorite; write their characteristics below (similar to terms in the tables)
Mineral
kyanite
chlorite
Characteristics (luster, color, hardness, cleavage, other features)
15. a. Look at rock sample R34, a regionally-metamorphosed shale. Name two
minerals that are in this rock. Hint: there’s a dark one and a light one.
b. Given that muscovite is present in R34 but hard to see, what grade of
metamorphism does this mineralogy imply (use table 6.1)?
c. Still using that table, what metamorphic facies is R34?
d. So what is the name of the rock? To find this, use the diagram below.
One way that metamorphic petrologists try to quantify the conditions of
metamorphism for various rocks is to draw a pressure/temperature (P/T)
diagram as shown in the figure on the next page. The field of the graph shows
the ranges of various metamorphic facies. The vertical axis shows the depth of the
metamorphism and the equivalent pressure in kilobars (kb). 1 bar is
approximately 1 atmosphere of pressure, and therefore 1 kb is about 1000
atmospheres of pressure. The horizontal axis shows the temperature of the
metamorphism in degrees Celsius.
16. a. Use the facies from question 4 to determine the range of possible
maximum pressures and the range of possible maximum temperatures at
which R34 formed. Use units of °C for temperature and kbar for pressure.
b. Suppose another area where the parent rock was found was subjected to less
than 1 kbar of pressure but the same temperature range during metamorphism.
Name one other mineral (besides the ones you named in the previous question)
you would expect to find.
As you have seen, some minerals are quite useful in determining the grade or type
of metamorphism because they can only form under certain metamorphic
conditions. These are called index minerals.
17. You are given the following information about a metamorphic rock:
Mineral composition: pyroxene, garnet, kyanite
Chemical composition: silicon dioxide 50.24%, aluminum oxide 13.32%,
calcium oxide 10.84%, iron oxide 9.85%, magnesium oxide 8.39%
Which type of composition is more useful in determining the grade and parent
rock of metamorphism and why? Or do both lists give equivalent information?
18. a. Now look at R35, which is the same metamorphic grade as R34. What are the
mineralogical differences? (In other words, what minerals show up in R34 but not
R35? In R35 but not R34?)
b. But what is the name of this rock, anyway? Hint: kind of a trick question.
19. In fact, for many metamorphic rocks, the most common mineral in the rock is
used as an adjective in front of the rock name. Fill in the appropriate mineral
name for the samples below, using the suggested test given:
Sample #
Test
Rock name
R34
Cleavage
_____________ schist
R35
Obvious mineral
_____________ schist
R36
Color
_____________ schist
R37
Scratch
_____________ schist
Parent
rock
shale
rhyolite
granite
basalt
limestone
sandstone
conglom.
Intensity of metamorphism
Low grade
grade
slate
High
phyllite
schist
gneiss
amphibolite
marble
quartzite
metaconglomerate
20. What changes in foliation thickness and mineral grain size would you
expect to see in a shale as it is subjected to greater temperatures and pressures
during metamorphism? (Hint: compare, in order, R38, R39, R34, R40)
21. So fill in the following rock names, using your answer to the previous question
and the fact that each sample represents a different metamorphic grade:
Sample #
Metamorphic grade
Rock name
R38
R39
R40
22. R41 and R42 are nonfoliated metamorphic rocks (they are sometimes called
“granoblastic rocks”); both of these rocks achieved the same grade of regional
metamorphism as R34 and R35 did. Identify the rock names using the hints
suggested in the characterization column; identify their parent rocks from the
table above.
Sample #
Characterization
R41
Glass plate
R42
Acid bottle
Rock name
Rock parent rock
Plate Tectonics and Metamorphic Rocks
23. R43 is blueschist, a unique type of metamorphic rock that forms under
conditions of high pressure and low temperature. Label the area on the crosssection below where you might expect blueschist to crystallize.
24. So, if you were to find blueschist as you walked along the Appalachian Trail in
North Carolina, what could you infer about the history of the East Coast of the US?
25. R44 is serpentinite, which blueschist often becomes over time. A key
mineral in blueschist is forsterite, a form of olivine, with the chemical formula
Mg2SiO4. A key mineral in serpentinite is (surprise) serpentine (chemical
formula: Mg3Si2O5(OH)4). How does serpentinite form from blueschist? (Hint:
consider readily available simple molecules at metamorphic depths and the
difference between the two chemical formulae)
26. R45 is hornfels, a unique type of metamorphic rock that forms under
conditions of low pressure and high temperature. Label the area on the crosssection below where you might expect hornfels to crystallize.
27. What is hornfels' parent rock? Or is there a unique parent rock?
28. Why is contact metamorphism such an appropriate term for this type of
metamorphism?
Geologic Map of Wyoming (1985)
29. Yellowstone NP is located in the northwest corner of the map; find a chunk of
Ti rock (it’s a magenta color) located at about 44.8° N, 109.8° W. Write the
description of the Ti rock, and, even though the explanation does not explicitly
state it, determine what the radiating lines of that Ti color are, geologically.
Finally, explain why contact metamorphism is likely in this area.
30. Recall that replacement of the skarn minerals is where the gold and other
metals will end up. To produce skarn, you need some carbonate rocks nearby –
rocks like limestone or dolomite. Look around at nearby formations from the Ti
outcrop, and, using the explanation sheet, determine the name(s) of the potential
carbonate formation(s) and the name(s) of the rock(s) they contain that will
metamorphose to skarn.