MINERALOGY
First In-class Exam
October 13, 2008
Name ______________________
Answer all questions. Their values are indicated next to the question. Closed book, closed
help, open mind. Cheating will result in a failing grade for the class. You have one hour.
(1) (10 points) For the first five minerals in the following list write the exact chemical formula
beside the mineral name. For the second five minerals in the list write the group of the mineral
(e.g. silicate, oxide, halide etc.). For these five minerals, you will get 0.5 extra credit points for
each correct major cation (excluding Si and Al) listed that makes up the mineral. You will lose
0.5 points for each incorrect cation listed.
Chemical Formulas:
(1) Forsterite
(2) Dolomite
(3) Anhydrite
(4) Gypsum
(5) Tridymite
Groups
(1) Phlogopite
(2) Cerrusite
(3) Witherite
(4) Strontianite
(5) Muscovite
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(2) (15 points) a) Define mineral.
b) List the eight most abundant elements making up earth’s crust. Abundances of two of these
element(s) would be much greater if the bulk earth were considered, rather than just the crust.
Which two are they and explain why they are different (e.g. where are they found if not in crustal
rocks?)
(3) (15 points) Assume the lengths of the sides of a unit cell for Timtebowite are a = 4 Å, b = 4
Å, and c = 2 Å. Axial intercepts are given for two faces in the mineral. Calculate the miller
indices for both faces. Be sure to show NEATLY your calculations.
a) Face A:
a=2Å
b=4Å
c=4Å
b ) Face B:
a=2Å
b=8Å
c=1Å
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(4) (20 points) List the six crystal systems. Describe or neatly draw the relative lengths of their
crystallographic axes and angles between the axes.
What determines the lengths and angular relationship between the crystallographic axes for any
particular mineral?
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(5) (15 points) a) Isotropic and anisotropic minerals usually display extinction differently.
Briefly describe how extinction may differ between the two types of minerals.
b) Explain why these two types of minerals have their characteristic extinction patterns.
c) In some orientations, anisotropic minerals may have exactly the same type of extinction as
isotropic minerals. Describe that orientation and explain why the mineral has the extinction that
it does.
(6) (15 points) Use the chart below to answer the following questions. Draw lines on the graph
to show how you arrived at the answer, and then write the answer in the space available,
including units where appropriate.
a) From interference colors, you determine quartz (=0.009) has retardation,  = 400 µm. What
is the thickness of the thin section?
b) An unknown mineral in the thin section from question a displays maximum interference
colors showing retardation of 1000 µm. What is its birefringence?
c) You find another grain which you think is the same mineral that you observed in question 6b,
but it has different interference colors than the first grain. Explain how two grains of the same
mineral may have different interference colors.
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