Geo 306 lab 02 Name __________________________________________________
p. 1 of 8
Optical Properties of minerals and petrographic techniques
Goals for this lab:
1. Learn the simplest observations/tests for mineral identification in thin section: cleavage, habit,
color/pleochroism, birefringence, relief. (Note that other techniques, such as elongation and extinction
angle, will be covered later during relevant mineral topics)
2. Learn the relationship between birefringence, thickness, and interference color.
3. Learn to efficiently make these observations.
4. Learn how to use a petrographic microscope.
5. Become familiar with the most common minerals/mineral groups in thin section: quartz, feldspar,
calcite, micas, amphiboles, pyroxenes.
Lab lecture topics
1. Microscope etiquette.
2. Parts of the microscope.
3. How to focus at high power.
4. What the accessory plate is used for.
5. What you are not learning (conoscopic techniques).
6. How this lab will run.
Lab groups: You will be sharing microscopes, and can work together, but should turn in individual
work on this lab.
Instructions for this exercise
You should feel free to skip around somewhat, in order to complete the lab in a timely manner. I’ve put a
microscope cabinet key and on reserve for you with Vicki (in ES 240) so you can have microscope access
outside class time. Be sure that all microscopes are locked when you are finished with them.
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”. In Geology 407, you will begin with
the interaction of electromagnetic waves with crystal structures, and work your way up from there to achieve a
detailed understanding of why these recipes work.
You should refer frequently to your textbook 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: first focus up while
watching the gap between the objective and the thin section (from the side). When it
is small, then look through the microscope and focus down.
Geo 306 lab 2 p. 2 of 8
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. Be sure you can locate the following parts of your microscope. Your instructor will demonstrate, and there
is a diagram on p. 119. Note that our scopes are not of identical design compared to the scope in the text,
but the parts are all present.
ocular objective lens stage
analyzer (upper polarizer) Bertrand lens stage vernier
condensing lens accessory opening stage goniometer
lower polarizer cross hairs aperture diaphragm
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.
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 magnification. Also, using a ruler (low & medium power) or stage
micrometer (high power), measure the total field of view.
(ocular x objective)
Diameter of field of
4. Your upper polarizing sheet (the “analyzer”) is oriented at 90° to the lower polarizer. 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
Geo 306 lab 2 p. 3 of 8
Plane Polarized Light Techniques – Relief (Nesse, p. 152-4)
5. 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 grain mounts of fluorite, sodalite, halite, beryl, and garnet to answer
the questions below (do not use your text!).
Which mineral shows the most relief? _____________________________________________________
Which mineral shows the least relief? _____________________________________________________
Which minerals have positive relief? ______________________________________________________
Which minerals have negative relief?______________________________________________________
Put these minerals in order from lowest to highest index of refraction (n). _________________________
Plane Polarized Light Techniques – Pleochroism & Color (136-7)
In general, color is more reliable in thin section than 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.
Some minerals change color as you rotate the stage in plane-polarized light. This is called pleochroism; it
can often be useful.
6. Thin Section W89 shows tourmaline, a strongly pleochroic mineral. Look up the typical shape of
tourmaline grains in your textbook.
Describe the shape: ___________________________________________________________________
In a thin section, the grains will be in all sorts of orientations relative to the plane of the slide, so there will
be different shapes shown.
Sketch an elongate grain shape, and a compact grain shape below.
For each grain, label the color or colors it shows in plane-polarized light as you rotate the stage. This
variation is pleochroism.
Which grain shapes show the most pleochroism? ____________________________________________
Geo 306 lab 2 p. 4 of 8
7. 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 two grains, each of which
shows a different pair of these colors (some will be blue & yellow, some blue & purple, some purple &
Sketch each grain below in two orientations, each labeled with the colors displayed.
Plane Polarized Light Techniques – Cleavage (p. 121)
Note that some of the grains you have seen show cleavage (e.g., halite, fluorite). 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. These marks can be more easily observed when the iris
diaphragm is reduced to enhance contrast.
8. Thin section SN14 has prominent crystals of hornblende, an amphibole (pleochroic green to light brown).
Find a grain that’s not elongate, and which displays the distinctive amphibole cleavage (2 cleavage
direction at about 60° to each other).
Sketch this grain below, showing the cleavage. (Remember, the outline of the crystal has not been
broken in a thin section – you will have to rely on internal crack patterns.)
9. Thin section IPu37 has olivine, which are the large low-relief grains that have bright colors in cross-
polarized light (with the analyzer in the light path).
Recall your hand specimen lab – what is the breakage behavior of olivine (you can look it up in your
text if you don’t recall)? _________________________________________________________________
Sketch a typical olivine grain that shows this breakage pattern.
Geo 306 lab 2 p. 5 of 8
Cross-Polarized Light Techniques – Introduction
10. Up until now, you have mainly been working in plane-polarized light (PP), with the analyzer out of the light
path. Cross-polarized light offers a new set of powerful techniques for mineral identification. First, using
section IPu37, find an olivine grain (see above question for identifying olivine).
What is the color of olivine (color = apparent color in PP)? ____________________________________
Insert the analyzer. Now you are in XP. The color you observe for the grain is called the interference
color (if it’s dark or black, rotate the stage until it’s at its brightest).
What is the interference color for this grain? ________________________________________________
Find another grain of olivine. What is the interference color for this grain? ________________________
What can you conclude about the relationship between color (PP) and interference color (XP)? _______
Take this grain, and rotate the stage.
Describe how the grain’s appearance changes in XP as you rotate the stage over 360°. Note what
changes -- is it the hue of the color, or is it something else?: ___________________________________
The way that the grain goes to black every 90° is called extinction; nearly all minerals show it.
11. Move to a part of the thin section near the edge, where there is no rock, only glass.
Describe what glass looks like in PP:______________________________________________________
Describe what glass looks like in XP (be sure to rotate the stage to see if it changes): _______________
You should remember these properties – often, during thin section production, a grain may be plucked
out of the rock, and only a mineral-shaped region of glass and epoxy left behind to distract you. To
make matters worse, the epoxy can collect dust-sized mineral particles created by the grinding process,
which can make it look very much like a mineral in PP.
In PP light, find a grain that is black (opaque). These grains are probably magnetite.
Describe what magnetite looks like in PP and XP: ___________________________________________
What is the luster of magnetite (look it up)? _________________________________________________
These properties are generally those of materials with lusters of this general type.
12. In thin section MBr38 there are sparse grains of almandine garnet. They can be found most easily in XP,
because they are black.
Describe what garnet looks like in PP and XP: ______________________________________________
What is the crystal system of garnet? (look it up; it’s the word just below the generalized mineral
These properties are typical of minerals in this crystal system. What can you conclude about the use of
interference color to discriminate among minerals in this crystal system? _________________________
Geo 306 lab 2 p. 6 of 8
Cross-Polarized Light Techniques – Identifying Interference color (126-7)
13. Examine the interference color chart in the back of your textbook. This is the set of interference colors
shown by most minerals. Along the bottom is the path difference, also called retardation. Note the red
dots along the bottom.
These red dots appear at what values of path difference (in nm)? _______________________________
These dots divide the chart into regions we call orders. So everything up to and including the first red
makes up the first order. From the blue at about 600 nm up to and including the next red is the second
order. When we name an interference color, we cannot just state the color, we must include the order
as well. Thus, an interference color might be “second-order blue”, or “fourth-order green”. Note that
some colors are absent from some orders. This odd progression of colors represents optical processes
the occur in the microscope to produce interference colors, but the explanation is too complex for this
course – take Geo 407.
Name an impossible interference color, such as “fourth-order blue”: _____________________________
14. Get thin section Quartz Wedge. This is an unusual type of thin section, which is thinner on one end than
the other (ideally, thin sections are 30 µm thick everywhere, although in practice the edges are frequently
Compare the progression of colors in the thin section to the interference color chart: ________________
15. In thin section PM026A, find a grain of muscovite. These are common, small elongate grains of moderate
relief which are colorless in PP and colorful in XP. Look around for a grain with very tiny concentric stripes
of color near the edge.
What interference color, generally, is shown by the center of this grain? (don’t worry about the order
just yet) _____________________________________________________________________________
In order to figure out which order this color represents, you have two methods:
Method One – Thin edges: Frequently, the edge of a mineral grain is not perpendicular to the stage,
but slopes at an angle. Because of this, near the edge of a grain, the mineral gets progressively
thinner. By choosing a grain with stripes, you chose a grain for which this was true. You can use these
stripes to figure out the order of the interference color, because as a grain gets thinner, the interference
colors shown will move smoothly down the chart towards white, then gray, then black.
If you have a grain showing second-order blue, what progression of interference colors would you
expect to observe at its edges? __________________________________________________________
Method Two – Accessory plate: By inserting the accessory plate, one of two things will happen.
Either the interference color of a grain will jump up a whole order, or down a whole order. If the
interference color jumped up, then just rotate the stage 90°, and the colors will jump down, which is
what you want. By decreasing the color by a whole order, you can often recognize the color more
easily. Note that if the color decreases past black, you should imagine a mirror-image chart on the
negative side (so decreasing an order from light grey produces yellow, and increasing an order from
light grey produces blue).
If you have a grain showing third-order blue, and the interference color is reduced by an order, then
what will be the new interference color? ___________________________________________________
If you have a grain showing second-order blue, and the interference color is reduced by an order, then
what will be the new interference color? ___________________________________________________
Geo 306 lab 2 p. 7 of 8
Explain how this example would allow you to use the accessory plate to figure out the order of a grain
showing a blue interference color.
Getting back to your muscovite grain. use the thin edge method to determine the interference color
(including order) for this grain. ___________________________________________________________
What is the path difference for this color? __________________________________________________
Use the accessory plate and describe the color you observe when the accessory plate is inserted so
that the color jumps down an order. _______________________________________________________
Remove the accessory plate now.
16. Note that as the interference colors increase, they get paler, more pastel, and are reduced to just pink and
green by fourth order. Eventually, (after about 6th order) the colors merge into a pale tan. We call this color
“high-order white”. Get thin section SS021B. It contains calcite, which shows this high-order white in XP.
You can find it because it often has bars of rainbow-ish colors going through it in two directions.
Find a grain of calcite and describe the progression of colors in XP as you rotate the stage (focus on
the main part of the grain, not the colored bars): _____________________________________________
Most of the low-relief grains in PM026A are quartz. Find one of these grains that has a white
interference color. Describe any difference between the look of calcite’s high-order white and quartz’
first-order white. ______________________________________________________________________
A useful way to tell these apart, if you can’t just see the difference, is to use the accessory plate.
For the calcite grain, when it’s 45° away from extinction, what happens when you insert the accessory
For the quartz grain, when it’s 45° away from extinction, what happens when you insert the accessory
Explain how you can use the accessory plate to distinguish first-order white from high-order white
Cross-Polarized Light Techniques – Birefringence (126-7)
17. Examine thin section I-S-21.
Describe the range of interference colors, including order, shown by the various quartz grains in I-S-21.
Describe the range of interference colors, including order, shown by the various biotite grains in I-S-21.
Geo 306 lab 2 p. 8 of 8
The variation in colors you observe is due to the orientation of the grain relative to the thin section
(remember the tourmaline example?). Any mineral can show an interference color of black if it’s in the
appropriate orientation (you probably didn’t find a black biotite, because it’s very unlikely for a biotite to
be in the exact needed orientation to show black, but you might have found a first-order orange one).
The distinctive property of a mineral in XP is not the interference color shown by any particular grain,
but the highest interference color displayed by the set of grains of that mineral.
What is the highest interference color shown by quartz in I-S-21? _______________________________
What is the path difference that corresponds to this interference color? __________________________
What is the highest interference color shown by biotite in I-S-21? _______________________________
What is the path difference that corresponds to this interference color? __________________________
18. In order to determine birefringence of a mineral in a thin section, you merely have to find the maximum
interference color shown by that mineral in the thin section (as in the exercise 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 but 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: path difference =
birefringence x thickness (note that path difference is typically in nm, while thickness is typically in
µm, so units must be adjusted for this equation to work)
Assume that the thin sections are all of standard thickness, 30 µm. If so, then:
What is the birefringence for quartz in I-S-21?_______________________________________________
What is the birefringence for biotite in I-S-21? _______________________________________________
What is the birefringence for olivine in IPu37?_______________________________________________
What is the birefringence for muscovite in PM026A? _________________________________________
How do these compare to the listed values for birefringence given in your text? (Note that there is a
typo in the book’s value for biotite, which should read 0.03-0.07)________________________________
19. 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.
Epidote with 25% Octahedral Fe3+ (use fig. 15.4) ____________________________________________