ASBOG Mineralogy and Petrology 2 — Questions and Answers
Question 1: Which crystal system is characterized by three unequal axes, all intersecting at oblique angles?
- Triclinic (Correct answer)
- Monoclinic
- Orthorhombic
- Trigonal
Correct answer: Triclinic
The triclinic system has three unequal axes (a≠b≠c) with all three axial angles differing from 90°.
Question 2: A rock containing plagioclase, hornblende, and biotite with a porphyritic texture most likely formed as a:
- Hypabyssal intrusive igneous rock (Correct answer)
- Deep plutonic igneous rock
- High-grade metamorphic rock
- Chemical sedimentary rock
Correct answer: Hypabyssal intrusive igneous rock
Porphyritic texture with those minerals indicates a hypabyssal (shallow intrusive) or volcanic rock where phenocrysts grew slowly before rapid cooling.
Question 3: The mineral sphalerite is the primary ore of which metal?
- Zinc (Correct answer)
- Lead
- Copper
- Silver
Correct answer: Zinc
Sphalerite (ZnS) is the principal ore mineral of zinc and commonly occurs with galena in hydrothermal deposits.
Question 4: Which metamorphic index mineral first appears at the lowest grade in Barrow's zones of regional metamorphism?
- Chlorite (Correct answer)
- Biotite
- Garnet
- Kyanite
Correct answer: Chlorite
Chlorite defines the lowest-grade Barrovian zone, appearing before biotite, garnet, staurolite, kyanite, and sillimanite with increasing P-T conditions.
Question 5: Orbicular texture in igneous rocks refers to:
- Spherical masses of concentric mineralogic shells (Correct answer)
- Random distribution of phenocrysts in a fine matrix
- Alternating light and dark mineral bands
- Rounded xenoliths enclosed in magma
Correct answer: Spherical masses of concentric mineralogic shells
Orbicular texture consists of spheroidal structures with concentric layers of alternating mineral compositions, found in some granites and diorites.
Question 6: The mineral aragonite and calcite are best described as:
- Polymorphs (Correct answer)
- Isomorphs
- Pseudomorphs
- Allomorphs
Correct answer: Polymorphs
Aragonite and calcite are polymorphs — both are CaCO₃ but with different crystal structures (orthorhombic vs. trigonal).
Question 7: An ultramafic rock composed almost entirely of olivine is called:
- Dunite (Correct answer)
- Peridotite
- Pyroxenite
- Harzburgite
Correct answer: Dunite
Dunite contains >90% olivine and represents the most olivine-rich end-member of the peridotite family.
ASBOG Mineralogy and Petrology 2
Which crystal system is characterized by three unequal axes, all intersecting at oblique angles?