V. ISOTROPIC MINERALS

Isotropic minerals are easy to recognize in thin section because they are the only group to go extinct under crossed polarized light (analyzer in). In plane polarized light (analyzer out), these minerals can be identified by using properties such as cleavage, relief, and color. Although there are at least 25 naturally occurring isotropic minerals, we will only look at a few of the more common ones in lab.

Spinel Group: (Mg, Fe+2, Zn,Mn) Al2O4
Kerr, p, 230-32
7a-5, [10]

The spinel group contains a large number of minerals including optically opaque magnetite and chromite (nearly opaque). The generally transparent minerals of the spinel series of the spinel group are represented in the slides. The spinel series are often deep green, but other colors are also possible.

a) Look carefully at spinel slides for a dark green mineral--this is spinel (MgAl2O4). What is the relief of spinel? Is the spinel mostly euhedral or anhedral? What is its textural relationship to other minerals in this slide?

b) Look at the chromite slide (M147 [10] describe the chromite (relief, color, form, etc.)

Volcanic Glass:
Kerr, p. 469
BH-250-9 [6] and BH51 [9]

Look at a variety of thin sections and make notes on characteristics as follows:
a) What is the relief of the glass with respect to the mounting medium? Is the R.I. higher or lower in the glass.
b) What color is the glass?
c) Scan the entire slide and note the overall rock texture. Does the glass look uniform across the slide, or is there evidence of either a fragmental texture (e.g. a pyroclastic rock) or other textures?
d) Are there any characteristic structures within the glass such as fracture pattern, mineral inclusions, or vesicles (bubbles)?

Pervoskite: CaTiO3
Kerr, p. 233
'Perovskite' [7]

Examine the slides of Perovskite (1,2). It is a relatively rare mineral. It only occurs in silica poor rocks. This is governed by the reaction CaTiO3 + SiO2 = CaTiSiO5 (sphene), thus it can never occur with quartz (1,2,3,4,5,6). Perovskite (1,2) is listed as pseudo isotropic so it will not go completely extinct under crossed polars. Look at the relief of Perovskite (1,2), n = 2.34. This is one of the highest relief minerals you will see.

a) Note the patterns displayed on the mineral when you cross the polarizers. How do they change with rotation of the stage? What are they?

Garnet group: (Ca, Mg, Fe, Mn)3 (Al, Cr, Fe)2 (SiO4)3
Kerr, p. 387-389
BH-250-2 [6], BH-250-3 , BH-250-10 [6], BH-250-20 [6], BH-250-26 [6], and BH58 [6]

Garnet (1,2,3,4,5,6,7,8,9,10,11) occurs in most commonly in metamorphic rocks although it can also occur in granites. There are many different types of garnets. Certain properties such as color, density and refractive index are unique to each type of garnet.
a) What is the relief of garnet? How does it compare to spinel or perovskite (1,2)?
b) Garnet (1,2,3,4,5,6,7,8,9,10,11) is commonly porphyroblastic in metamorphic rocks. Note the little inclusions in the garnets. What do these tell you about the way Garnet (1,2,3,4,5,6,7,8,9,10,11) grows?

Leucite: KAlSi2O6
Kerr, page 319
BH53 [7]

Leucite is a potassium rich mineral that is found in potassium rich, silica undersaturated volcanic rocks, commonly as large distinctive euhedral phenocrysts. Check the relief and R.I. compared to the balsam. Examine these slides carefully and be sure you can locate the Leucite (1,2,3). How can you be sure this isotropic groundmass material is Leucite (1,2,3)? How can you differentiate Leucite (1,2,3) from Analcite?

Note:
1. some of the leucite (1,2,3) show a reaction rim. What is a reaction rim?
2. There is Titanaugite (1,2) in these thin-sections. What is a titanaugite (1,2)?

Fluorite: CaF2
Kerr, p. 222
M69-SC [9], BH50 (10)

Examine the slides and find the Fluorite. What are the typical shapes of the mineral in this rock? Is it euhedral, subhedral or anhedral? What is the relief with respect to quartz (1,2,3,4,5,6)? Is the cleavage obvious? How many directions?

Sodolite: 3NaAlSiO4.NaCl
Kerr 424
BH-250-33 [6]

Examine the slides of sodolate. This is a common mineral in Soda-Rich igneous rocks such as syenites and trachytes.

Haüyne: 3NaAlSiO4 (CaSO4)
Kerr, p. 324
"Haüyne" [1]

Haüyne is a blue feldspathoid of the sodalite group. Haüyne occurs in soda-rich volcanic rocks such as phonolites and contact metamorphic limestones.
a) Look at the thin section that contains this mineral and study its properties.
For exercise

For exercise:

1. Study all the isotropic minerals discussed in lab.
3. Obtain a volcanic glass from your instructor and determine the RI of the mineral (glass), using the different oils that we have in the lab. Using the camera mounted on the microscope, take pictures of the becke lines and turn them with your RI determination.

4. Identify the isotropic minerals in the thin-sections provided by your instructor. Include all properties that helped you identify these minerals.

Tactics for identifying minerals in thin-section
Important:
-Do not blindly collect optical information hoping that you will magically produce a match. Instead, use each piece of information to:
+ Eliminate possible minerals.
1. Use tables in book
2. Write down, or keep in mind, as many alternative minerals (candidates) as you can.

+Determine what optical information you need next.
1. What information will eliminate yet more minerals?
2. Try to identify the information that will eliminate the most minerals possible, then obtain it.

+Determine what tests are necessary and how much help they will be in obtaining the information you want.
1. Do not waste time doing unnecessary tests!
An example: Looking at many uniaxial mineral grains in conoscopic view to try to find a centered optic axis figure without first looking for grains with minimum birefringence.
________________________________________________________________________
TOOLS YOU ALREADY HAVE/NEED.
-Use Mineral Associations to help determine unknown minerals. (handout given in class)
-Use Crystallographic systems and models to:

1. Explain what is seen in thin-section.
2. Predict what possible shapes, cleavages, and orientations a particular mineral will have in the section.
Note: Procedure outlined in the textbook and in these notes include many useful tests for determining optical information. To use these tests in the right way, and without redundancy, means that you need to understand how each test works (let your TA or instructor know if you don't understand a test!).

Tactics:

1. Plane Polarized light
Start in plane polarized light on low to medium power (objective) with the light source not at maximum brightness. Scan the slide and get a rough feel for what is present (i.e., how many different minerals, texture, veins, alteration, metamorphic, igneous, sedimentary or anything that might help). Cross and uncross the polars and rotate the stage as needed to record the following information (usually starting off with one mineral):

a. Color and Pleochroism (if any).
b. Relief (low, moderate, high--relative to balsam n ~ 1.54).
-if possible, determine whether relief is negative or positive.
c. Mineral grain shape, texture (if any), and alteration (if any)
- anhedral, subhedral, or euhedral?
d. Is the mineral Isotropic or anisotropic.
e. Is twinning present? If so what type? (Use cross polars to check).
f. Cleavage, Fractures.

2. If the mineral in question is isotropic:

a. Make sure!!! Check for the absence of an interference figure and make sure all grains go extinct under cross-polars.
b. Go to the identification charts and try to determine the mineral from the properties listed above.

3. If the mineral in question is anisotropic:

a. Scan the slide for the grain with the lowest interference color.
Obtain an optic axis figure (if possible) then determine whether the mineral is uniaxial or biaxial.

4. If the mineral is Uniaxial:

a. Conoscopic-Determine optic sign using optic axis figure test or flash figure test . Use the same grain(s), used in 3(a), with lowest birefringence.

b. Orthoscopic-Record color (PPLand XPL) and relief of nw. Check Becke line to estimate nw. Once again, use the grain, or grains, with the lowest birefringence (looking down c-axis).

c. Find a grain with the highest birefringence color. Determine the maximum birefringence of the mineral using color chart (assuming that the thickness of the thin-section is standard). Place ne parallel to the slow ray of the accessory (ROT-1) plate, then determine the color (PPLand XPL) and relief of ne.

d. If the mineral is elongated or has cleavage:
-Determine sign of elongation (length slow or length fast)
-If crystallographic direction can be identified-determine optic axis orientation (where is the C-axis?). Use flash figure test or other tests as needed. Compare results with the known crystal model.
-Record orientation of cleavage (parallel to C-axis, etc.) angle between cleavages (if more than one), and how well developed (poor, fair to good, very good, and perfect cleavages).

5. If the mineral is biaxial:
Use the same procedure as for uniaxial BUT with the modification below.

a. Find a centered optic axis figure (look for low birefringence grain).Determine optic sign.
It is possible to use an acute bisectrix to find the sign. (Obtuse bisectrix will give you a different optic sign, so you have to make sure if you are looking down the x or z axis.

b. Determine 2V. (See handout to estimate the 2V).

c. Record Dispersion (type and degree if any).

d. Return to orthoscopic view, record the color (PPL) and relief of the grain (Gives value of nb).

e. Find a grain with Maximum birefringence:
-Using optic figures and the accessory plate, determine the fast and slow directions (na and ng ), their respective reliefs, & the pleochroic colors (if any) associated with X and Z.

f. If the mineral has cleavage:
-Measure extinction angles (0-120o) on several grains and take the maximum value.

g. If the mineral is elongated determine sign of elongation.

----------CONSTANTLY MATCH OPTIC INFORMATION TO TABLES----------

**** SEE PAGE 181 IN KERR FOR KEY TO MINERAL TABLES, THEN USE THEM!! ****

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