ASBOG Mineralogy and Petrology 3

Question 1

Which property best distinguishes dolomite from calcite in the field?

Accepted Answer

Dolomite fizzes weakly only in powdered form with cold HCl

Answer

Dolomite reacts sluggishly with cold dilute HCl only when powdered, whereas calcite fizzes vigorously with cold acid even in massive form.

Question 2

In the IUGS classification of plutonic rocks, a rock with 60% plagioclase, 30% pyroxene, and 10% olivine would be classified as:

Accepted Answer

Norite or gabbro

Answer

A rock with that mafic mineral content (pyroxene + olivine) and calcic plagioclase plots in the gabbro/norite field of the QAPF diagram.

Question 3

The characteristic luster of metallic sulfides like pyrite and chalcopyrite is called:

Accepted Answer

Metallic luster

Answer

Metallic luster describes the shiny, opaque reflective quality of sulfide minerals that resembles polished metal.

Question 4

Eclogite is a high-pressure metamorphic rock primarily composed of:

Accepted Answer

Omphacite pyroxene and pyrope-rich garnet

Answer

Eclogite is defined by the assemblage of omphacite (Na-Al pyroxene) and pyrope-rich garnet, forming under very high pressures in subduction zones.

Question 5

Which igneous rock texture describes a fine-grained groundmass with no visible crystals?

Accepted Answer

Aphanitic

Answer

Aphanitic texture occurs when magma cools rapidly, producing crystals too small to see with the naked eye.

ASBOG - Association of State Boards of Geology Practice Test