ASBOG Mineralogy and Petrology 3 — Questions and Answers
Question 1: Which property best distinguishes dolomite from calcite in the field?
- Dolomite fizzes weakly only in powdered form with cold HCl (Correct answer)
- Dolomite has a higher hardness than calcite
- Dolomite displays perfect cubic cleavage
- Dolomite is always white while calcite is gray
Correct answer: Dolomite fizzes weakly only in powdered form with cold HCl
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:
- Norite or gabbro (Correct answer)
- Diorite
- Anorthosite
- Troctolite
Correct answer: Norite or gabbro
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:
- Metallic luster (Correct answer)
- Adamantine luster
- Resinous luster
- Vitreous luster
Correct answer: Metallic luster
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:
- Omphacite pyroxene and pyrope-rich garnet (Correct answer)
- Kyanite and quartz
- Glaucophane and lawsonite
- Jadeite and albite
Correct answer: Omphacite pyroxene and pyrope-rich garnet
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?
- Aphanitic (Correct answer)
- Phaneritic
- Pegmatitic
- Glassy
Correct answer: Aphanitic
Aphanitic texture occurs when magma cools rapidly, producing crystals too small to see with the naked eye.
Question 6: The term 'foliation' in metamorphic rocks refers to:
- Planar fabric defined by aligned platy minerals or compositional layering (Correct answer)
- Random orientation of minerals in a massive rock
- Alternating coarse and fine layers of sedimentary origin
- Fractures produced by tectonic stress
Correct answer: Planar fabric defined by aligned platy minerals or compositional layering
Foliation is the planar structural fabric in metamorphic rocks created by preferred orientation of platy minerals like mica or by compositional banding.
Question 7: Pseudomorphism occurs when:
- One mineral replaces another while retaining the original crystal shape (Correct answer)
- Two minerals crystallize simultaneously from the same melt
- A mineral displays two different crystal forms
- A mineral develops cleavage parallel to a previous crystal face
Correct answer: One mineral replaces another while retaining the original crystal shape
A pseudomorph retains the external form of an original mineral while the internal composition has been replaced by a different mineral.
ASBOG Mineralogy and Petrology 3
Which property best distinguishes dolomite from calcite in the field?