ASBOG Mineralogy and Petrology 5 — Questions and Answers
Question 1: The Al₂SiO₅ polymorphs kyanite, sillimanite, and andalusite are distinguished from each other primarily by:
- Pressure and temperature conditions of formation (Correct answer)
- Chemical composition differences
- Country of geological occurrence
- Associated igneous rock type
Correct answer: Pressure and temperature conditions of formation
The three Al₂SiO₅ polymorphs have identical chemistry but form under different P-T conditions: andalusite (low P), kyanite (high P), and sillimanite (high T).
Question 2: Which rock type is produced by the solidification of lava flows on the seafloor, characterized by pillow-shaped lobes?
- Pillow basalt (Correct answer)
- Komatiite
- Flood basalt
- Trachyte
Correct answer: Pillow basalt
Pillow basalt forms when submarine lava is rapidly chilled by seawater, creating ellipsoidal to pillow-shaped lobes with glassy rinds.
Question 3: The Mohs hardness scale assigns a hardness of 7 to which mineral, commonly used as a standard?
- Quartz (Correct answer)
- Feldspar
- Topaz
- Corundum
Correct answer: Quartz
Quartz (SiO₂) defines hardness 7 on the Mohs scale, scratching glass and most common minerals but being scratched by topaz (8), corundum (9), and diamond (10).
Question 4: In the classification of metamorphic rocks, a gneiss differs from a schist by:
- Having compositional banding rather than continuous foliation defined by platy minerals (Correct answer)
- Containing more quartz than feldspar
- Being formed only at very high pressures
- Lacking any mica minerals
Correct answer: Having compositional banding rather than continuous foliation defined by platy minerals
Gneiss is characterized by compositional banding (alternating felsic and mafic layers) and coarser grain size, while schist has continuous foliation defined by abundant aligned mica.
Question 5: Bowen's Reaction Series describes which process?
- The sequential crystallization of minerals from a cooling basaltic magma (Correct answer)
- The order in which minerals dissolve during weathering
- The metamorphic grade sequence from low to high temperature
- The chemical evolution of seawater through geologic time
Correct answer: The sequential crystallization of minerals from a cooling basaltic magma
Bowen's Reaction Series outlines the continuous and discontinuous branches of mineral crystallization as basaltic magma cools, predicting which minerals form first.
Question 6: A mineral with a specific gravity of approximately 19.3 and a yellow metallic luster that does not tarnish is most likely:
- Native gold (Correct answer)
- Pyrite
- Chalcopyrite
- Marcasite
Correct answer: Native gold
Native gold has a specific gravity of ~19.3, a rich yellow metallic luster, and resists tarnishing and chemical attack, distinguishing it from fool's gold (pyrite, SG ~5).
Question 7: Which process best explains the formation of a migmatite?
- Partial melting of metamorphic rock producing interlayered melt and solid portions (Correct answer)
- Injection of granite veins into an unrelated host rock
- Chemical replacement of carbonate by silica
- Mechanical mixing of two distinct rock types during faulting
Correct answer: Partial melting of metamorphic rock producing interlayered melt and solid portions
Migmatites form at the transition between metamorphism and magmatism when high-grade metasedimentary or metaigneous rocks begin to partially melt, creating leucosome (melt) and melanosome (residue) layers.
ASBOG Mineralogy and Petrology 5
The Al₂SiO₅ polymorphs kyanite, sillimanite, and andalusite are distinguished from each other primarily by: