Geo 306 Fall 2016 lab 02
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Name __________________________________________________
Optical Properties of minerals and petrographic techniques KEY
Lab groups: You will be sharing microscopes, and can work together, but should turn in individual
work on this lab.
Instructions for this exercise
This lab will not be set in the “jigsaw” format, but in a more traditional format. I’ve provided you with a short
primer on petrographic techniques, which covers more than we will cover in this course – it may be useful in
407, if you take that course.
There are not enough sections for everybody to work on the same ones at the same time. You should feel free
to skip around somewhat, in order to complete the lab in a timely manner. I’ve put a scope cabinet key and on
reserve for you with Vicki (in ES 240) so you can have microscope access outside class time.
Introduction
The optical properties of minerals can be extremely valuable not only for mineral identification, but for
understanding rock history. For example, optical techniques can be useful in deciding at what temperature a
rock has been deformed. Due to time constraints, we will not be able to provide a theoretical framework for
optical petrography – we will be treating these techniques as a “black box” or a recipe, instead of beginning
with the interaction of electromagnetic waves with crystal structures, and working our way up from there with a
detailed understanding of why these recipes work.
You should refer frequently to your textbook (and the Petrography Guide, if you wish) while working on this lab.
Microscope setup & parts
1. You will first be assigned a microscope.
Microscopes are very expensive, and ours are old and somewhat fragile. You must
use care at all times, when using and handling them. You should only pick them up
by the strong, curved "arm" and also hold them under the base. They should remain
at the desk near their cabinet.
You should in particular be careful about focusing at high power (read Nesse, p. 121).
Do not grab an objective lens to change magnification – instead, grab the knurled ring of mount to which
the objectives are screwed to switch objectives.
2. Refer to Fig 7.9 to locate the following parts of your microscope. Note that our scopes are not of identical
design compared to the scope in the text, but the parts are all present.
ocular
objective
stage
analyzer (upper polarizer)
Bertrand lens
stage vernier
condenser
accessory opening
stage goniometer
lower iris diaphragm
cross hairs
aperture diaphragm
lower polarizer
ocular micrometer
Note that for all parts of this lab but the last, you should not have the accessory plate inserted. Remove it now
from the scope and put it on the table.
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Read about focusing in Nesse (p. 121). Note that it is easy to damage the high-power objective! In order
to avoid this, you should always be in focus using the medium power objective, and then move to high power.
The sample should be nearly in focus, and you should use the fine adjustment knob to achieve focus. If the
sample is not nearly in focus, then raise the stage while viewing the sample from the side of the scope to avoid
crushing the thin section with the objective, and then bring the specimen into focus by lowering the stage
only.
3. For each objective, record the N.A. from the lens markings, calculate the total magnification. Also, using a
ruler (low & medium power) or stage micrometer (high power), measure the total field of view.
Objective
Magnification
2.5X
10X
40X
Total magnification
(ocular x objective)
25X
100X
400X
Diameter of field of
view
~ 7mm
~1.8mm
~0.5mm
4. Using a biotite-bearing thin section (JT88-6e, W124), determine and record the vibration plane of the
polarizer (lower polarizing sheet). Here’s how: biotite absorbs light that vibrates parallel to its cleavage
direction. So, with the analyzer (upper polarizing sheet) removed from the light path, rotate the stage until
the biotite is darkest. If this is East-West (compass directions are usually used to describe orientations in
the microscope view; E-W means horizontal), then that is the vibration direction of the polarizer. If this is
N-S, then that is the vibration direction.
Vibration direction of polarizer (N-S or E-W) _________________________________________ E-W
5. Your upper polarizing sheet (the analyzer) is oriented at 90° to this direction. When the analyzer is not in
the light path, the polars are said to be "uncrossed" and the illumination is "plane polarized light" ("PP").
When the analyzer is in the light path, the polars are "crossed" and the illumination is "cross-polarized light"
("XP"). Get a thin section of a rock and see what the difference is. Try a few. Enjoy the pretty colors.
6. In the exercises above, you were using the microscope in orthoscopic illumination mode. To set up the
microscope for conoscopic illumination mode, you do the following:
1. Cross the polars
2. Go to high-power magnification and focus on a sample. Be sure that the sample stays under the
center cross-hairs as you rotate the stage. (If not, ask Dave to center your objective lens).
3. Insert the condenser and the Betrand Lens. The weird thing you see is called an interference figure.
Note that we will not be using conoscopic techniques in this class, but you will learn them if you take Geo 407.
Here’s an example of how to acquire an “interference figure”, the data one collects using conoscopic
techniques:
Get an oriented thin section of calcite (perpendicular to c) and try to get an interference figure. Sketch it very
briefly below (it should not look like a mineral grain). Observe what happens differently if you use medium or
low power instead. What if you uncross the polars?
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Plane Polarized Light Techniques – Relief (Nesse, p. 152-4)
7. Relief is related to n, the index of refraction. The amount of surface texture tells you how different nmineral is
from 1.54, (the index of refraction of the thin section cement). The Becke Line test tells you whether nmineral
is more or less than 1.54. Examine the thin sections of fluorite, sodalite, halite, beryl, and garnet to answer
the questions below.
Which mineral shows the most relief? ____________________________________________ garnet
Which mineral shows the least relief? ______________________________________ halite or beryl
Which mineral shows the highest (most positive) relief? ______________________________ garnet
Which mineral shows the lowest (most negative) relief? _____________________________ fluorite
Put these in order from lowest n to highest n. ____________ fluorite, sodalite, halite, beryl, garnet
Plane Polarized Light Techniques – Pleochroism & Color (136-7)
In general, color is not as unreliable in thin section as it is in hand specimen (although it’s still not the best
diagnostic tool). Minerals that are opaque in thin section (typically oxides, hydroxides, and sulfides) are usually
black or metallic in hand specimen. Usually, the color in thin section is much lighter than the color in hand
specimen.
Pleochroism (color that changes as you rotate the stage, as in the biotite you used above to figure out the
vibration direction of your lower polarizer) can often be useful.
8. Thin Section W89 shows tourmaline, a strongly pleochroic mineral. Tourmaline is in the hexagonal crystal
system. Note that some grains show an elongate shape, and some show an equant (equidimensional)
shape with the typical tourmaline cross-section (a “puffy” triangle, or a distorted hexagon, depending on
how you look at it). Think about the polarized light coming up from the polarizer.
What color is the light vibrating parallel to the c-axis? ____________________________ light brown
What color is the light vibrating perpendicular to the c-axis? _______________________ dark brown
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9. Glaucophane (ALP-10, or “Glaucophane”) has three different pleochroic colors: pale yellow, blue, and
purple. Any individual grain, however, will only display two of the three. Find three grains, each of which
shows a different pair of these colors (one will be blue & yellow, one blue & purple, one purple & yellow).
Sketch each grain below, showing the vibration direction for each of the two colors in each grain.
blue
yellow
blue
purple
yellow
purple
Plane Polarized Light Techniques – Cleavage (p. 121)
Note that some of the grains you have seen show cleavage (think biotite, fluorite) that is: grain margins that are
smooth and straight with parallel opposite sides. When cleavage is seen, there are often also small incipient
fractures within the grain that appear as a series of short dark lines parallel to each other and the straight
external boundaries.
10. Thin section 179-KS 27 has prominent crystals of augite (weakly
pleochroic pale pink to pale green), a clinopyroxene. Find a grain
that displays the best example of pyroxene cleavage (2 cleavage
direction at about 90° to each other). Good grains will probably be
equant and more or less octagonal. Sketch this grain below,
showing the cleavage.
11. Thin sections JT89-5, JT886g both contain garnet
(probably near almandine
composition). These are the
pink
roundish
high-relief
grains that are black in cross-polarized light.
Recall your hand specimen lab – what is the breakage behavior of
garnet? conchoidal fracture
Sketch a typical garnet grain that shows this breakage pattern.
Cross-Polarized Light Techniques – Interference color (126-7)
Interference color is the color displayed by a grain in cross-polarized light, when the stage is not rotated so that
the grain is black. For most purposes, the key feature will be the maximum interference color displayed by a
given type of grain throughout a thin section. When we describe a color, we state the order and color together,
so it might be “second-order blue”, or “fourth-order green”. Note that some colors are absent from some
orders, and that the orders end just above the red/pink value. So the lowest red in the chart is “first-order red”,
and there is no such thing as “first-order blue”.
12. Choose a thin section labeled “Quartz || c”. In XP, note the interference color in the center of the grain.
Refer to the Interference Color Chart in the back of Nesse, which links the observed color to a retardation
value.
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What retardation is shown by this grain? ______________________________________ 300-350 nm
(Note that the color decreases from the middle of the grain towards the edges. This is because the grain
gets thinner there. This will be a useful feature in a task below.)
13. As stated above, the most important interference color for a mineral in a thin section is the maximum
interference color displayed by any grain of that mineral. Thin section RC-8 has four minerals that make up
most of the rock: biotite (reddish brown), hornblende (green), plagioclase (colorless, but striped in XP), and
pyroxene, probably enstatite (high-relief, pale pink to green pleochroism).
What is the highest interference color displayed by the orthopyroxene?________ First-order yellow
What retardation is shown by this grain? _________________________________________ 350 nm
14. Thin section 20-35 contains magnetite (opaque octahedra with squarish outline) and olivine phenocrysts
both “swimming” in a “sea” of clinopyroxene. This is a cumulate texture, which forms when olivine and
magnetite crystals settle out of a magma, and then the pyroxene crystallizes around them. To determine
an interference color for the olivine, you need to know what order of color you are seeing. To tell this, you
should closely examine the edges of the grain, where you can see the color decrease on the thinning edge.
This allows you to judge what order of color you are seeing in the main body of the grain.
What is the highest interference color displayed by the olivine? ________ Third-order green-yellow
What retardation is shown by this grain? ________________________________________ 1350 nm
15. In minerals with a strong color (in PP light) it can be difficult, or impossible to judge interference color. Try
it in the biotite (thin section W124)
What is the highest interference color displayed by the biotite? _________________Third-order red
What retardation is shown by this grain? ________________________________________ 1600 nm
Cross-Polarized Light Techniques – Birefringence (126-7)
In order to determine birefringence for a mineral in a thin section, you merely have to find the maximum
interference color shown (as in the exercises above), and then use the Interference Color Chart to find the
birefringence. One important assumption, however, is the thickness of the section. The standard thickness is
30 µm, however this is often not quite achieved, or is achieved only in part of the section (one edge thin the
other thick, or the center thick and the edges thin). You can use a known mineral such as quartz or
plagioclase, with a known birefringence, to judge the thickness of a thin section or a region of a thin section.
There is a mathematical formula for birefringence which can be useful as well:
retardation = birefringence x thickness (note that retardation is typically in nm, while thickness is typically in
µm, so units must be adjusted for this equation to work)
16. Assume that the thin sections you examined above were all of standard thickness, 30 µm. If so, then:
What is the birefringence for orthopyroxene in RC-8? _____________350E-9 / 3E-5 = 1.1E-2 = 0.011
What is the birefringence for olivine in 20-35? _______________________________________ 0.045
What is the birefringence for biotite in W124? _______________________________________ 0.053
How do these compare to the listed values for birefringence given in your text? ________________
_______________________________________________________________________________
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_______________________________________________________________________________
Can the birefringence value tell you anything about the composition of the minerals? yes, see the solid
solution chart for olivine as an example
17. Use the birefringence values in your text, together with the interference color chart or the equation above to
determine the maximum interference color you would expect to observe for the following minerals in a
standard thickness thin section/
Muscovite _________________________________________________________ mid-fourth order
Quartz ___________________________________________________________ first-order yellow
Plagioclase ________________________________________________________ first-order white
Hornblende _______________________________________________________ second-order red
Epidote with 25% Octahedral Fe3+ (use fig. 15.4) ________________________________________
Garnet (note that garnet is isometric) __ (there are no interference colors for isometric minerals)
Calcite ?!? (Hey how can you figure that out? What’s going on with that? (don’t answer; these are
rhetorical questions. Continue with the lab, and it will be made clear.)
18. Get a thin section of calcite (“Marble”) and pick a random grain. Describe what the interference color of
this grain looks like to you.
should be pale pink or tan with hints of other colors
19. .In XP, insert the accessory plate into the slot in the scope.
Does the interference color change? _____ No, assuming the grain was not extinct to begin with.
Try the same thing with quartz. Does its interference color change? ______________________ Yes
Still with quartz, with the plate in, rotate the stage 360°. Describe the color variation It goes from pink (at
extinction) to blue to pink to yellow and back to pink again.
Try that with calcite (plate in, rotate stage). Describe the color variation It goes from pink (at extinction)
to tan to pink to tan.
What you are seeing is the difference between first-order white (well, actually, quartz is yellowish, but you get
the idea, right?) and “high-order white”, which is the pastel color we observe for very high birefringence
minerals in most orientations.