ASBOG Mineralogy and Petrology 4 — Questions and Answers
Question 1: The silica tetrahedron (SiO₄⁴⁻) in olivine group minerals is arranged as:
- Isolated tetrahedra with no shared oxygens (Correct answer)
- Single chains sharing two oxygens per tetrahedron
- Double chains sharing alternating oxygens
- Sheet structures with three shared oxygens
Correct answer: Isolated tetrahedra with no shared oxygens
Olivine is a nesosilicate (island silicate) where each SiO₄ tetrahedron shares no oxygens with neighboring tetrahedra.
Question 2: Which sedimentary rock, upon contact metamorphism, typically recrystallizes to form marble?
- Limestone or dolostone (Correct answer)
- Shale
- Sandstone
- Chert
Correct answer: Limestone or dolostone
Limestone and dolostone are carbonate rocks that recrystallize to marble under heat and pressure, coarsening the calcite or dolomite grains.
Question 3: In igneous petrology, the term 'cumulate' refers to rocks formed by:
- Crystal settling or accumulation from a magma chamber (Correct answer)
- Partial melting of the mantle
- Assimilation of wall rock by magma
- Mixing of two distinct magma batches
Correct answer: Crystal settling or accumulation from a magma chamber
Cumulate rocks form when early-crystallizing minerals sink or float in a magma chamber and accumulate into distinct layers.
Question 4: Which mineral group includes muscovite, biotite, and phlogopite?
- Micas (Correct answer)
- Amphiboles
- Pyroxenes
- Feldspars
Correct answer: Micas
Muscovite, biotite, and phlogopite are all sheet silicates belonging to the mica group, characterized by perfect basal cleavage.
Question 5: A granodiorite differs from a true granite primarily in having:
- More plagioclase than alkali feldspar (Correct answer)
- More alkali feldspar than plagioclase
- Higher mafic mineral content than tonalite
- No quartz in its composition
Correct answer: More plagioclase than alkali feldspar
Granodiorite contains more plagioclase than alkali feldspar (P/(A+P) ratio 65–90%), while granite has more alkali feldspar.
Question 6: The mineral magnetite (Fe₃O₄) belongs to which mineral class?
- Oxides (Correct answer)
- Sulfides
- Carbonates
- Silicates
Correct answer: Oxides
Magnetite is an iron oxide mineral (spinel group) classified in the oxide mineral class, strongly magnetic and commonly found in mafic igneous rocks.
Question 7: Pressure solution in metamorphic rocks occurs when:
- Minerals dissolve preferentially at grain-to-grain stress concentrations (Correct answer)
- Fluids introduce new minerals into a rock
- Minerals recrystallize without change in composition
- Temperature causes partial melting at grain boundaries
Correct answer: Minerals dissolve preferentially at grain-to-grain stress concentrations
Pressure solution is a deformation mechanism where minerals dissolve at high-stress contact points and may reprecipitate elsewhere, leading to grain compaction and stylolites.
ASBOG Mineralogy and Petrology 4
The silica tetrahedron (SiO₄⁴⁻) in olivine group minerals is arranged as: