You will have 50 minutes to complete this 100 point exam. Please use your time accordingly. Use the space beneath the question, or the back of the facing page to answer questions. Show your work (and logical outline) where possible. For essay and short answer questions use concise but complete sentences and paragraphs, and good grammar. You may use your notes and texts, but ask questions only of the proctor.
1) 7pts. A highly birefringent mineral has high relief and is a phenocryst in basalt. It is optically negative with a 2V angle of 83 degrees. What is the mineral and its composition?
2) 7pts. Within a gabbro you find large crystals of a mineral with two different cleavages at 90 degrees to each other. The birefringence and relief are low. The 2V angle is 80 degrees and optically negative. It has simple and polysynthetic twins with parallel composition planes. What is the mineral?
3) 7pts. Within a schist you find prismatic crystals with distinctive pleochroism {X=colorless, Y=green, Z=green}. The crystals are too small to measure cleavage angles, but in the hand sample you can see that the mineral develops a nice prismatic cleavage. What is the mineral?
4) 7pts. Within a sandstone are small detrital grains with distinctive basal cleavage which controls their orientation (parallel to depositional bedding). In hand sample the crystals are transparent, and in thin section the retardation is first-order. What is the mineral?
5) 7pts. Within a sandstone are many grains with perfect hexagonal cross-sections which yield uniaxial positive interference figures. The grains have low relief and low birefringence. What is the mineral? Would it represent detritus from volcanic, plutonic, or metamorphic terraines?
6) 10pts. In a plutonic rock are crystals with 90 degree cleavages that show parallel or symmetric (not inclined) extinction, high relief (n over 1.75), but only upper first-order retardation. It is optically positive with 2V of 87 degrees. What is the mineral and its composition?
7) 10pts. In well-cuttings you find prismatic crystals with 120 degree cleavages. In thin section the pleochroism is X=colorless, Y=blue, Z=violet. What is the mineral? What is the significance of finding it?
8) 15pts. A thin section of sandstone has abundant grains of feldspar and quartz. You need to determine if the source of the sand was plutonic or volcanic. What features would you look for in the feldspars and quartz? (use back of page for complete answer)
9) 15pts. Give the exact chemical formula for:
a) diopside
b) an olivine with 50 mole % forsterite, 50 mole % fayalite
c) muscovite
d) anorthite
e) tridymite
10) 15pts. What mineral is represented by the following formula:
a) SiO2
b) Na0.8K0.2AlSi3O8
c) Mg3Si4O10(OH)2
d) Mg1.8Fe0.2SiO4
e) Mg2Si2O6