GEOL 2082            Exam #2             Spring 1996

1. 24 pts. Discuss the nature and significance of the types of polyhedral structures and linkages that produce the variations seen in phyllosilicate minerals. Discuss how these affect optical and other physical properties as well as the crystal chemisty of the minerals. You should begin with something like Figure 14.1 - from your textbook, Nesse (1991). You may include references to figures in your textbooks and you may use your own small drawings, but this is principally an essay. Confine your answer to two pages. You should use a typewriter or word-processor, but limit your answer to two pages.
 

2. 6 pts. Olivines.

a) What is the expected composition of an olivine which has a 2V angle of 85 degrees? What additional information would you need to know to in order to give a precise answer?

b) Calcium also occurs as a stoichiometric constituent in monticellite and other (Fe, Mg, Ca) orthosilicates with an olivine structure. Discuss the nature of the calcium substitution and the limitations it imposes on the compositions of these minerals.

3. Pyroxenes.

a) 5 pts. Describe the physical appearance and optical properties of the cleavage that characterizes the orthopyroxenes in this group. You might begin with the crystal sketch on page 205 of Nesse (1991)

b) 2 pts. Enstatite and bronzite are the two common orthopyroxenes? What is the range in observed birefringence and 2V angle in these two?

4. 3 pts. Which amphiboles are:

a) typical of igneous rocks?

b) most common in low-grade metamorphic rocks?

c) characteristic of high pressure metamorphism?

 5. 4 pts. Phyllosilicates.

a) What does dioctahedral mean?

b) What is the common igneous phyllosilicate?

6. 12 pts. Feldspars.

a) How are Albite and Carlsbad twins distinguished?

b) Characterize the physical appearance and optical properties of exsolution.

c) Ordering of Al and Si in specific tetrahedral sites in the potassium feldspars produces what effects?

7. 6 pts. What is the relationship between polymorphism, crystal symmetry, physical properties and temperature in the silica minerals?

8. 8 pts. Very short answers.

a) The formula NaAlSi2O6 is that of ______________________.

b) Untwinned feldspar is difficult to distinguish from __________ in thin section.

c) The mineral jadeite has the __________ optic direction parallel to the b-axis.

d) The patchy or tartan pattern in _________ is due to twinning by the albite and pericline laws.

9. 4 pts. What is the stoichiometric formula for the plagioclase that has 50% each of albite and anorthite?

10. 4 pts. Sketch the interference figure of a biotite flake mounted such that the cleavage is flat on the glass slide and the a-axis is oriented NE.

11. 4 pts. Use the following measurements to determine the composition of a volcanic plagioclase by the Carlsbad-Albite Method:

Extinction angles for albite twins left of Carlsbad twin: 34, 36° Extinction angles for albite twins right of Carlsbad twin: 24, 26° 12. 4 pts. What orthopyroxene (in % MgSiO3) has the following measured properties:

max. birefringence - 0.0185

2V angle - 75°

optically negative

13. 2 pts. At higher temperatures plagioclase will have more or less potassium feldspar in solid solution?

14. 2 pts. Mystery mineral: Strongly colored brown and pleochroic; basal cleavage; no or rare twins; 2V is 5°.

15. 2 pts. Mystery mineral: first-order retardation; indices all near than 1.53; polysynthetic twinning; two unequal cleavages at about 90°; triclinic.

16. 8 pts. What mineral does each formula represent:

Na2Mg3Al2Si8O22(OH)2

NaAl2(AlSi3)O10(OH)2

NaFeSi2O6

NaAlSi3O8