1. 25 pts. Discuss the nature and significance of the types of tetrahedral linkages that produce the variations seen in silicate minerals. Especially discuss how this effects optical and other physical properties you observe in thin section and hand sample. You should begin with something like Figure 13.3 - silicate classification - in Klein and Hurlbut’s Manual of Mineralogy. 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 one page.

2. 4 pts. Olivines. What would the expected maximum birefringence be for an olivine that was 20 mole % forsterite?

3. Pyroxenes. 5 pts. Identify the pyroxene with parallel extinction (though symmetric viewed down the c-axis) and with negative 2V at 70 degrees. Birefringence is above 0.015.

4. 6 pts. Amphiboles that are:

a) typical of low-grade, greenschist metamorphism?

b) most common in igneous rocks?

5. 10 pts. Phyllosilicates.

a) What layering scheme characterizes talc?

a) What is the ratio of tetrahedral aluminum to silicon in the true micas?

6. 10 pts. Feldspars.

a) How are Albite and Carlsbad twins distinguished?

b) Discuss the significance of ordering of Al and Si in specific tetrahedral sites in the potassium feldspars.

7. 4 pts. Silica.

a) What is the fibrous form of quartz?

b) What is the hydrous SiO₂ material found in sedimentary rocks.

8. 8 pts. Very short answers.

a) The formula NaAlSi₂O₆ is that of ______________________.

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

c) Strong absorption colors including browns, yellow-browns, and red-browns are common in the amphiboles called ________________.

d) The patchy or tartan pattern in microcline is due to twinning by the ______________ and ______________ laws.

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

10. 6 pts. Calculate the weight percents of MgO, SiO2, and FeO in an olivine whose mole proportions are forsterite 60% and fayalite 40%.

11. 4 pts. Use the following measurements to determine the composition of a volcanic plagioclase:

14. 12 pts. What mineral does each formula represent: