Geos 356 Petrology
Spring 2003
Lab 2
Properties observed in cross polarized light: isotropism, anisotropism, interference
colors, uniaxial and biaxial interference figures
Today we will concentrate on the properties that are studied in XPL. The first big distinction that we make while
observing in XPL is between ISOTROPIC and ANISOTROPIC minerals. Isotropic means “same in all directions”, i.e.
every property displayed by an isotropic mineral is the same regardless of the direction of measurement.
Exercise 1.1: List the 6 crystallographic crystal systems in the two classes isotropic or anisotropic and justify your
answer
ISOTROPISM
Exercise 1.2
TS min30 or 74-501-25 or min102 or min103 or 10-20-59-1
Everybody should take a look at min30
You worked on TS................
Scan the TS in PPL and look for a euhedral-subeuhedral (six sided) or anhedral (depending on the TS) mineral
with high relief, it might have some inclusions, no pleochroism and with color in shades of red-pink (very light), no
cleavage and some fractures. Verify for yourself all the characteristics previously mentioned. This mineral is garnet.
Now switch on XPL, what happens?
Do you observe any change as you rotate the stage?
Make a sketch of garnet in PPL and XPL.
Exercise 1.4
TS min135 or K65
In PPL concentrate on the spinel, the mineral that displays green color (min135) or gray color (K65); give its relief,
pleochroic scheme if any, cleavage or fracture, shape (whether the mineral is euhedral or anhedral), alteration, zoning…
Now switch to XPL, what happens to the mineral you have been observing?
See any changes by rotating the stage?
Lab 2`
Page 1
Geos 356 Petrology
Spring 2003
What you have just observed for garnet and spinel is called isotropism. The minerals appeared transparent in PPL
but after inserting the analyzer they appeared dark regardless of rotation of the microscopic stage.
Remember from the first lab the look of opaque minerals. What is the most obvious difference between opaques and
isotropic minerals under a microscope?
Is it correct to say that opaques are isotropic minerals?
Exercise 1.3
to do after exercises 1.2 and/or 1.3
TS min1 or min4 or 92:9 or min2
You worked on TS..................
The mineral in the TS’s that we are going to observe is leucite. Look for sub-euhedral grains (more or less
spherical), transparent with no color, some fractures and negative relief (the RI is so much lower then the rest of the
minerals or the glue, that leucite appears buried in the TS; instead of “standing out” it “stands in”). Try to see how negative
relief appears. Call for assistance if necessary.
When finished with observation in PPL switch to XPL and rotate the stage. How does leucite appear?
Draw a sketch of leucite in XPL
Is there any difference between the intensity of darkness for spinel or garnet and leucite?
Minerals like leucite are called pseudo-isotropic. Can you explain why?
ANISOTROPISM
We have already noticed that most minerals display a variety of colors when the analyzer is inserted. This is known
as interference color and is due to the fact that one light ray entering the minerals breaks up into two rays travelling at two
different velocities. The minerals showing interference colors are called anisotropic minerals.
Because of the difference of their velocities, an anisotropic mineral exhibits two different RI-s for the two (split-up) rays.
The difference between the RI’s of the two rays depends on the optic orientation of the TS (depends on the orientation of
the minerals). The maximum difference of RI’s that a mineral can exhibit is known as its birefringence. The difference
between RI’s in any arbitrary section - other than the one that shows max birefringence - is known as apparent birefringence.
The interference colors exhibited by any mineral are a function of different variables:
Interference colors (retardation) = thickness * (N-n)
Where thickness is the thickness of the TS, usually at 30 microns
N = max RI
n = min RI
(N-n) = apparent birefringence or birefringence
Every mineral displays a particular birefringence, and the interference colors are very diagnostic. You have to keep in mind
though that in a given TS the same mineral can display different interference colors with respect of the orientation of the
mineral. All these interference colors can be equal or less than the one allowed by the max birefringence, but never higher.
Lab 2`
Page 2
Geos 356 Petrology
Spring 2003
There are charts where the max birefringence is translated into interference colors as a function of the thickness of the TS.
We have several of these charts in our lab.
Please take a look at one of them and reproduce a sketch of it in the following space. I do not need to see the colors on it.
The purpose of the exercise is to have you able to understand and read the chart.
Summarizing, we have learned that the interference color that a mineral in TS exhibits, depends on (a) its apparent
birefringence (which is function of the true birefringence and optic orientation) and (b) the thickness of the section. The
standard thickness of a TS is 0.03 mm or 30 microns. The IC-s reported for different minerals in optical mineralogy books
are based on this standard thickens.
The IC-s are divided into first, second, third or higher order groups and the IC of a mineral is indicated giving its order and
color, i.e. first order yellow. The color red (look at your chart) indicates the change of order.
Exercise 2.1
TS 3(a) or 7-5-61-17
You worked on TS.........................
These TS-s are composed mainly of plagioclase. This mineral displays, in 30micron TS - first order gray-white
interference colors. You have to learn how it looks like both in PPL and XPL. Look in your textbook and read about the
optical properties of plagioclase.
What other characteristic feature is displayed by plagioclase?
Look around the TS do you see other minerals other than plagioclase?
If yes, what is their max interference color (for now just compare the color you see with the one reported on the charts)?
When you rotate the stage what happens to the plagioclase?
Concentrate on a plagioclase lamella.
How many times does it go black upon 360o rotation?
When a mineral grain becomes black in XPL upon rotation it is said that the mineral is in ‘extinct’ position.
EXERCISE 2.2
TS min114 or min118 or 275-2 or 104
You worked on TS
These TS-s are mainly monomineralic. Look at the TS first in PPL and describe what you see. Pick a grain and follow the
scheme given on the last page of this lab.
Switch to XPL and give the max interference color displayed by the most abundant mineral species.
Rotate the stage and indicate how many times the crystal goes extinct.
Lab 2`
Page 3
Geos 356 Petrology
Spring 2003
This mineral is quartz, very common in almost every type of rock. You saw it a little bit last lab, and it is
important to learn how it looks like both in PPL and XPL. Check in your mineralogy textbook for a complete description of
the optical properties of quartz. Since this is a very common mineral, you will have many chances to identify quartz. When
the TS is 30 micron thick, the highest interference color is 1st order gray to yellowish gray. Check on the interference color
chart and give the correspondent max birefringence
What will be the IC for quartz if the TS was 45 micron thick?
You see that thickness is very important to assess the IC and vice-versa. But how can we make sure that the TS has a
standard thickness?
A common method of determining the thickness of a TS involves finding a mineral which can be identified without an
accurate knowledge of its IC, and then comparing its observed interference color with the thickness reported of the chart.
Quartz and plagioclase are such minerals. If they display non standard interference color, TS is not standard thickness.
Determination of the interference color
You might have already realized how difficult it is to find out the order of the IC-s just by comparison with what
we perceive and what is reported on the chart. There other methods that can help us in this task. We are going to learn just
one that will be useful in most cases
Technique number 1:
Recall the definition of interference color:
IC = thickness * (N-n)
We have already learned how thickness is important. Now if the same mineral has areas with different thickness,
what do you think will happen? Do you expect to see a uniform color or patches of different color with respect to the
different thickness? If your answer was the second one, you won! Our first technique develops along this principle: in most
grain mounts and TS grains tend to be thinner around their edges and, in effect, a proportion of the color chart is displayed
along wedge-like edges. If you count how many times the color red appears on the wedge you can figure out the order of the
IC very easily. Then you are left with simply identifying the main color on the mineral. And there you have it.
Exercise 2.4
TS text48 or min61 or min84 or min85 or k65
You worked on TS
Look for the mineral that shows fractures and possibly alteration of serpentine along them, rounded shape, no pleochroism,
is transparent and colorless and has high relief.
This mineral is olivine.
Draw a sketch of it.
In XPL look around the TS and try to locate a grain that displays the highest interference color and give its order and color
Repeat the measurement on more than one grain
Exercise 2.5
We will see that some minerals (phyllosilicates and calcite for example) show very high interference colors. When IC’s are
high they lose definition and look like pastel color instead of being very bright. In these next TS you will look at calcite.
Please try to remember the look of high interference colors
Lab 2`
Page 4
Geos 356 Petrology
Spring 2003
TS min31 or 5-31-63-2 or min25
You worked on TS........
Look for the mineral with high positive relief, cleavage traces, beige in color.
Observe it in XPL and rotate the stage.
What order of IC do you observe?
Does the mineral ever go extinct (becomes homogeneously dark)?
Describe the look of the mineral when it reaches its darkest color
This mineral is calcite. Check in your textbook for its optical description. You need to be able to recognize this mineral if it
comes back in other TS.
Uniaxial Minerals
Uniaxial Interference Figures
An interference figure allows you to determine (i) whether an anisotropic mineral is uniaxial or biaxial and (ii) the
optic sign. The following procedure is used to obtain an interference figure. (Your microscope must be perfectly centered)
1.
2.
3.
4.
5.
Cross the polars
Find a grain with very low birefringence
Focus on the mineral grain with the high-power objective.
Flip in the condenser
Insert the Bertrand lens.
If the optic axis (c-axis) is oriented perpendicular to the stage, the interference figure will look like Figure 2.6 in
your lab manual.
Isogyres are formed where the vibration directions in the interference figure correspond to the vibration directions of the
polarizer and analyzer, respectively. If the optic axis is perfectly vertical, the optic axis figure (OA figure, shown on p.15)
will not move or change when the stage is rotated.
Isochromes are bands of interference colors. Their number depends on the birefringence of the mineral and on the thickness
of the thin section.
Exercise 3: (2 green envelopes) In order to familiarize yourself with possible uniaxial interference figures, obtain
different types of interference figures using sections of quartz mounted in various orientations, and draw a sketch of the
different figures. Ask your TA for assistance!
(1) OA figure
(perpendicular to c-axis)
(2) Off center OA figure (quartz F)
(3) Flash figure (parallel to c-axis)
Lab 2`
Page 5
Geos 356 Petrology
Spring 2003
SUPPLEMENTAL NOTES:
Anisotropic Minerals : In the terminology of optical mineralogy, isometric crystals are isotropic; all non-isometric
minerals are anisotropic. In this lab, we will examine uniaxial minerals, which is one of the two classes of anisotropic
minerals. Uniaxial minerals belong to the hexagonal and tetragonal systems. When light enters an anisotropic mineral, its
velocity (and therefore its refractive index) depends on its direction of travel within that mineral. Since tetragonal minerals
have two (and hexagonal, three) equivalent crystallographic axes, light vibrating perpendicular to the c-axis travels at the
same speed in all directions, but it travels at a different speed at any other orientation relative to the c-axis. In uniaxial
minerals, the c-axis is referred to as the optic axis; grains cut perpendicular to the optic axis appear isotropic.
Birefringence: When light is split into two rays upon entering an anisotropic mineral, the two rays propagate at different
velocities. The light ray with the higher velocity (and therefore experiencing the lower RI) is called the fast ray. The ray
with the lower velocity (and experiencing the higher RI) is called the slow ray. The difference in the refractive indices
experienced by these two rays is called birefringence and is designated by the lower-case Greek letter delta ().
 = | RIs-RIf |
Interference: Anisotropic minerals viewed under crossed polars often show vivid colors. These so-called interference
colors are produced when the two rays of different velocities go out of phase while passing through the mineral.
Retardation is the distance D the slow ray lags behind the fast ray; the interference colors produced depend on retardation.
The interference chart shows retardation and interference colors. Interference colors occur in a repeating sequence from
blue to red at retardations of 551, 1101, 1652 nanometers, also referred to as first order interference colors (<551), second
order interference colors (551-1100) and so on. High order colors become more washed out and degenerate into a creamy
white color. The interference chart can also be used to determine birefringence because birefringence () and retardation
(D) are related by the equation:
D = d(thickness of section) * 
Extinction: Examine a tourmaline grain with crossed polars. You will notice that unless the optic axis is vertical, the
minerals go dark -- extinct -- under crossed polars once in every 90° of rotation. Extinction occurs when one of the rays
into which light is split is oriented parallel to the polarizer: all the light passing through the polarizer is absorbed by the
analyzer and none of the light can pass through. If the stage is rotated so that 2 rays vibrate at 45° to the polarizer and
analyzer, a maximum value for both the slow and fast ray is seen and the mineral appears dark. The angle between the
length or cleavage of a mineral and the vibration direction of its 2 rays is known as the extinction angle and is useful in
identifying the mineral.
Biaxial Minerals and an introduction to minerals in rocks
Biaxial minerals are somewhat more complex than uniaxial minerals. Biaxial minerals have three crystal axes: a ≠
b ≠ c; three optical axes (not optic axes): X < Y < Z; and three refractive indices:  <  < . The refractive indices can be
represented by a biaxial indicatrix, which is an ellipsoid with three unequal principal radii. The term biaxial refers to the
two optic axes oriented perpendicular to the two circular sections contained within the ellipsoid. The angle between the two
optic axes is designated 2V.
It is important to distinguish crystal axes from optical axes. Crystal axes -- a, b and c -- represent the physical
dimensions of a mineral; they are measured in angstroms and are not necessarily perpendicular to one another. By contrast,
optical axes -- X, Y and Z -- represent vibration directions of light traveling through a mineral; they are proportional to the
refractive indices and are always perpendicular to one another. X is the fastest direction and represents (the smallest RI).
Z is the slowest direction and represents (the largest RI). The direction Y represents (the intermediate RI) and
corresponds to the intersection of the two circular sections. In orthorhombic minerals, the optical directions are parallel to
the crystal axes, but in any order -- i.e., in some minerals, a is parallel to X (the fastest direction), but in other minerals b or
c may be parallel to X. In monoclinic minerals, the plane a-c contains the plane defined by any two optical directions, but
neither a nor c is necessarily parallel to those directions; b is parallel to the third optical direction. In triclinic minerals, a, b
and c are not necessarily parallel to any of the optical directions.
Lab 2`
Page 6
Geos 356 Petrology
Figure 1.
Spring 2003
The biaxial indicatrix (+).
Two methods for measuring 2V:
Olivine forms a solid solution with complete substitution between the end members forsterite and fayalite. Measuring 2V is
one of the easiest ways of estimating olivine composition: pure fayalite is biaxial (-) and has a 2V of ~46°, while pure
forsterite is biaxial (+) and has a 2V of ~82°. 2V varies linearly and continuously between the two end-members. The
angle 2V can be estimated visually from the curvature of an isogyre from an optic axis figure as shown below:
Muscovite has a moderate 2V, and is a good example for another technique of measuring 2V angles of less than about
55°. 2V is measured by using an acute bisectrix (BxA) figure which is obtained by looking down the Z-axis (for
positive minerals; see Figure 1) or X-axis (for negative minerals). When the stage is rotated, the isogyres form a crude
cross and then separate. The maximum distance between the two isogyres is a measure of 2V as shown below.
BEWARE: the N.A. of your objective significantly affects the apparent separation you observe (see chart on cabinet).
15°
30°
n
 = 1.7
45°
60°
N.A. = 0.85
A third technique for estimating 2V is known as Kamb's method, which works well for mildly off-center BxA or BxO
figures. Though we are not formally teaching this technique, a description of it can be found taped to the wooden
cabinet at the back of the room. It is particularly useful with minerals with a high 2V.
Exercise 4.1: T.S. 7-5-61-4, 7-5-61-10 (2 thin sections), 76076:12 (missing), K-52 (missing) or R-54. These thinsections are taken from igneous rocks. Find an appropriately-oriented olivine crystal and draw the optic axis
interference figure in the circle below and provide the information requested.
Thin section #
Figure and sign
2V
Orientation of indicatrix
Lab 2`
Page 7
Geos 356 Petrology
Spring 2003
How do you know you have "an appropriately-oriented olivine crystal" for finding an optic axis figure?
What happens to the interference figure when the stage is rotated?
Look for evidence of alteration of the olivine (some thin sections are better than others). What kinds of alteration
minerals might you expect to find?
Exercise 4.2: T.S. mica (2 thin sections), "vite", or one of the large cleavage flakes. These samples are oriented with
the {001} cleavage perpendicular to the path of the light. Find and draw a centered BxA figure, showing the 2V angle
and colors with the gypsum plate in.
Thin section #
Figure and sign
2V (optional but recommended)
Orientation of indicatrix
Lab 2`
Page 8
Geos 356 Petrology
Spring 2003
Exercise 4.3: T.S. biotite (2 thin sections) or 5(b), or one of the phlogopite cleavage flakes. These samples are
oriented with the {001} cleavage perpendicular to the path of the light. Find and draw a centered BxA figure, showing
the 2V angle and colors with the gypsum plate in.
Thin section #
Figure and sign
2V (optional but rcommended)
Orientation of indicatrix
Lab 2`
Page 9
Geos 356 Petrology
Spring 2003
DO AT LEAST ONE OF EXERCISE 5.1, 5.2 or 5.3. IF YOU HAVE TIME, TRY TO LOOK
AT THE SAMPLES AND THIN SECTIONS FOR TWO OR ALL THREE OF THE
EXERCISES.
Exercise 5.1: 1 H.S.: SJA-6; 4 T.S. SJA-6
a. Examine the hand sample. How many major minerals are present?
Identify as many as you can and explain the criteria you used.
b. Quickly scan the thin section under both plane-polarized light (PPL) and cross-polarized light (XPL). Now, under
PPL, describe any minerals that stand out (e.g. because of their distinctive cleavage, color, etc.). You ought to come
up with at least 2 distinctive minerals (though there are more!) Be specific and list as many observations as you can.
c.
Describe the minerals you observed in part b, now under XPL. Also, describe the distinctive optical properties of
the abundant colorless (in PPL) mineral under XPL. Try to identify these minerals.
d.
Lab 2`
What kind of rock is this?
Page 10
Geos 356 Petrology
Spring 2003
Exercise 5.2: 1 H.S. “4”; 3 T.S. “4”
a. Examine the hand sample. How many major minerals are present?
Identify as many as you can and explain the criteria you used.
b. Quickly scan the thin section under both plane-polarized light (PPL) and cross-polarized light (XPL). Now, under
PPL, describe any minerals that stand out (e.g. because of their distinctive cleavage, color, etc.). Be specific and list
as many observations as you can.
c. Try to identify the other minerals in the section (those more easily distinguished in XPL) describing the properties
which aid in their identification in XPL.
d. Most of the remaining minerals are probably low relief, low birefrengence( 1st order gray to yellow), and lacking
any noteworthy habit or cleavage. But there are other features present. Describe at least two (2) optical tests that will
provide you with additional information.
e.
Lab 2`
What kind of rock is this?
Page 11
Geos 356 Petrology
Spring 2003
Exercise 5.3: 1 H.S. 350:25, T.S. 350:25 (2 thin sections)
a.
Examine the hand sample. How many major minerals are present?
Identify as many as you can and explain the criteria you used.
b. Quickly scan the thin section under both plane-polarized light (PPL) and cross-polarized light (XPL). Now, under
PPL, describe any minerals that stand out (e.g. because of their distinctive cleavage, color, etc.). Be specific and list
as many observations as you can.
c. Identify the two most distinctive minerals in this sample and describe the optical properties you used to identify
them (in PPL and XPL).
d.
Lab 2`
What kind of rock is this?
Page 12
Geos 356 Petrology
Spring 2003
Here are some good questions (in a logical order) to ask when looking at and identifying minerals
in thin section.
Make a sketch of the mineral you have selected and with reference to it
answer the following questions:
1) is it opaque or transparent?
2) is it euhedral or anhedral?
3) how is its relief compared to the neighbors?
4) is it altered?
5) does it have any color?
6) is it pleochroic? if yes determine the pleochroic scheme
7) does it have any cleavage? fracture?
8) if cleavage is present write how many directions and measure their angle
9) is it twinned?
10) is it zoned?
11) take a guess at what mineral it is.
12) do you think the rock is metamorphic, igneous or volcanic?
Lab 2`
Page 13