MINERALOGY
Second In-class Exam
November 21, 2008
Name ______________________
• Answer all questions. Their values are indicated next to the question.
• Closed book, closed help, open mind.
• Cheating will result in an F for the class.
(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) that you listed that
makes up the mineral (not including trace elements). You will lose 0.5 points for each
incorrect cation listed.
Chemical Formulas:
(1) Andalusite
(2) Microcline
(3) Pyrrhotite
(4) Sanidine
(5) Enstatite
Groups
(1) Cassiterite
(2) Corundum
(3) Rutile
(4) Spinel
(5) Magnetite
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(2) (20 points)
a). (15 points) On the ternary diagram below, plot the following compositions and label
each point with their numbers: (1) En20Fs70Wo10, (2) En55Fs40Wo5, (3)
CaMg0.75Fe0.25Si2O6, (4) CaMg0.33Fe0.67Si2O6, (5) Ca0.9Mg0.5Fe0.6Si2O6
En = Enstatite, Fe = Ferrosilite (Fe2Si2O6), Wo = Wollastonite (Ca2Si2O6)
b) (5 points) Write the stoichiometrically correct chemical formula for (1) and (2).
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(3) (20) Imagine you have a thin section of calcite (hexagonal) with one grain cut
perpendicular to the c crystallographic axis. The thin section is standard thickness.
a) (5 points) Describe how the grain would look as you rotate the stage with your upper
analyzer inserted.
b) (5 points) What are the birefringence and retardation shown in this grain?

=
=
c) (10 points) In the circle provided draw the interference figure you would see for this
particular grain. Label the melatope, isogyres, and isochromes. Include on the drawing
the directions of vibration and the indices of refraction for each direction in each
quadrant, at the melatope, and on the isogyres.
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(4) In a thin section, you observe a pleochroic mineral, of which there are multiple grains.
One grain remains extinct all the time and its color is light green under plane polarized
light. When viewed with crossed nicols, another grain has the highest inference colors of
all the grains of this mineral. In plane polarized light, this grain has two colors: light
brown and dark brown. Considering these observations answer the following questions.
a) (5 points) Is the mineral uniaxial or biaxial? Why?
b) (5 points) What index of refraction corresponds to the light green color?
c) (5 points) Briefly describe how you would determine the orientation of the principal
indices of refraction for the grain displaying the light and dark brown colors.
d) (5 points) Assuming the fastest index of refraction corresponds to the light brown
color, write the complete pleochroic formula for the mineral?
(5) The K-spars come as three major polymorphs
a) (5 points) List their names from most ordered to least ordered. Which one is stable at
high temperature and which one is stable at low temperature?
b) (5 points) How do the structures of the three polymorphs differ, i.e. what makes them
polymorphs?
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(6) (10points) Draw the crystallographic axes for a tetragonal mineral. Make it clear
which is longest and shortest and what the angular relationships are between the axes.
Next to your drawing of the crystallographic axis, draw its indicatrix assuming it is an
optically positive mineral. On the drawing of the indicatrix, label the optic axis and
circular section. Indicate how the principle vibration directions (i.e., the indicatrix axis)
correspond to the crystallographic axes.
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(6) (10 points) In the following box, sketch a one component phase diagram of the
Al2SiO5 polymorph minerals. Be sure to label the P – T ranges of the axes (including
units) and include the boundaries separating stability fields of the minerals. Label the
field with the names of the minerals that are stable under the P and T conditions of each
field.
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