ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers 2

Question 1

What is the Bowen's Reaction Series, and what does it predict about the order of mineral crystallization from a mafic magma?

Accepted Answer

It describes the sequential crystallization of minerals from a cooling mafic magma, with olivine and Ca-plagioclase forming first, followed by pyroxene, amphibole, biotite, and finally quartz and K-feldspar

Answer

It predicts that minerals crystallize in order of their hardness, with the hardest minerals forming first

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It describes the sequence of metamorphic reactions in pelitic rocks with increasing temperature and pressure

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It predicts the alteration sequence of minerals during hydrothermal weathering of granite

Question 2

What is the key mineralogical difference between a granite and a rhyolite?

Accepted Answer

They have the same mineral composition (quartz, feldspar, mica) but different crystal sizes due to different cooling rates — granite is coarse-grained (intrusive), rhyolite is fine-grained (extrusive)

Answer

Granite contains orthoclase and quartz; rhyolite contains plagioclase and no quartz

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Granite forms by metamorphism of shale; rhyolite forms by hydrothermal alteration of basalt

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Granite contains hornblende; rhyolite contains olivine

Question 3

Which mineral group is characterized by a sheet silicate (phyllosilicate) structure and is primarily responsible for the plasticity and swelling behavior of expandable clay soils?

Accepted Answer

Smectites (montmorillonite group)

Answer

Feldspars

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Pyroxenes

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Amphiboles

Question 4

What is the CIPW normative mineral calculation used for in igneous petrology?

Accepted Answer

Converting a bulk chemical analysis of an igneous rock into a theoretical mineral composition for comparison and classification purposes

Answer

Determining the pressure and temperature conditions of magma crystallization from mineral assemblages

Answer

Calculating the radioactive decay constants of U, Th, and K in a rock for geochronology

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Determining the degree of partial melting from a mantle source rock's composition

Question 5

What are eclogites, and in what tectonic setting do they typically form?

Accepted Answer

High-pressure, high-temperature metamorphic rocks composed of omphacite (Na-Al pyroxene) and pyrope-rich garnet, formed by deep subduction of crustal material

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Low-grade metamorphic rocks formed in blueschist facies conditions at mid-ocean ridges

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Contact metamorphic rocks formed by magmatic intrusion into carbonate country rock

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Hydrothermal alteration products of basalt found at mid-ocean ridge hydrothermal vents

Question 6

What is the difference between a concordant and a discordant igneous intrusion, and give examples of each?

Accepted Answer

Concordant intrusions (sills, laccoliths) are emplaced parallel to existing host rock layering; discordant intrusions (dikes, stocks, batholiths) cut across existing structures

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Concordant intrusions are felsic; discordant intrusions are mafic, regardless of their relationship to country rock structure

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Concordant intrusions cool faster because they have greater surface area contact with country rock; discordant intrusions cool slowly

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Concordant intrusions form in compression; discordant intrusions form in extensional tectonic settings