Geos 356 PetrologySpring 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?

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.

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.

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

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.Cross the polars

2. Find a grain with very low birefringence

3.Focus on the mineral grain with the high-power objective.

4. Flip in the condenser

5.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.

Isochromesarebands 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)

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): XYZ; 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 twoopticaxes 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.

Figure 1.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).

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 thin-sections 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.