ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers 2 — Questions and Answers
Question 1: What is the Bowen's Reaction Series, and what does it predict about the order of mineral crystallization from a mafic magma?
- It predicts that minerals crystallize in order of their hardness, with the hardest minerals forming first
- 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 (Correct answer)
- It describes the sequence of metamorphic reactions in pelitic rocks with increasing temperature and pressure
- It predicts the alteration sequence of minerals during hydrothermal weathering of granite
Correct 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
Bowen's Reaction Series describes the discontinuous (olivine→pyroxene→amphibole→biotite) and continuous (calcic→sodic plagioclase) crystallization sequences from cooling basaltic magma, explaining igneous rock mineralogy through fractional crystallization.
N.L. Bowen's Reaction Series (1922) explains the observed mineral assemblages in igneous rocks through two parallel branches. The discontinuous series progresses: olivine → pyroxene → amphibole → biotite, with each mineral reacting with the melt as it cools. The continuous series shows progressive change from calcium-rich plagioclase (anorthite) to sodium-rich plagioclase (albite). Both series converge at K-feldspar, muscovite, and quartz at low temperatures. Fractional crystallization (early minerals removed from contact with melt) explains how a single basaltic parent magma can produce a range of derivative compositions.
Question 2: What is the key mineralogical difference between a granite and a rhyolite?
- Granite contains orthoclase and quartz; rhyolite contains plagioclase and no quartz
- 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) (Correct answer)
- Granite forms by metamorphism of shale; rhyolite forms by hydrothermal alteration of basalt
- Granite contains hornblende; rhyolite contains olivine
Correct 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)
Granite and rhyolite are compositionally equivalent (both felsic, both contain quartz and feldspar) but differ in texture: granite cools slowly at depth producing large crystals, while rhyolite cools rapidly at the surface producing fine-grained or glassy rock.
The principle of 'plutonic equivalent' pairs in igneous petrology recognizes that compositionally equivalent rocks exist in coarse-grained intrusive and fine-grained extrusive forms. Granite (intrusive) ↔ Rhyolite (extrusive); Gabbro ↔ Basalt; Diorite ↔ Andesite. The mineralogy is essentially identical (quartz + K-feldspar + plagioclase ± biotite ± hornblende for felsic rocks), but texture differs because slow cooling at depth allows large crystal growth while rapid quenching at the surface produces fine grains or glass. Porphyritic textures reflect two stages of crystallization.
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?
- Feldspars
- Pyroxenes
- Smectites (montmorillonite group) (Correct answer)
- Amphiboles
Correct answer: Smectites (montmorillonite group)
Smectites (montmorillonite) are 2:1 expandable sheet silicates that absorb water molecules between their silica-alumina layers, causing dramatic volume changes and high plasticity in soils containing these minerals.
Smectite group clays (including montmorillonite, nontronite, saponite) have a 2:1 layer structure (two silica tetrahedral sheets sandwiching one alumina octahedral sheet) with a large interlayer space that can accommodate water molecules and exchangeable cations (Ca2+, Na+, Mg2+). Na-montmorillonite (sodium smectite) is particularly expansive, swelling to many times its dry volume when wetted. Kaolinite (1:1 structure) and illite (2:1 non-expanding) have much less swelling potential. Smectite-rich soils ('black cotton soils', Vertisols) cause significant foundation and pavement damage.
Question 4: What is the CIPW normative mineral calculation used for in igneous petrology?
- Determining the pressure and temperature conditions of magma crystallization from mineral assemblages
- Converting a bulk chemical analysis of an igneous rock into a theoretical mineral composition for comparison and classification purposes (Correct answer)
- Calculating the radioactive decay constants of U, Th, and K in a rock for geochronology
- Determining the degree of partial melting from a mantle source rock's composition
Correct answer: Converting a bulk chemical analysis of an igneous rock into a theoretical mineral composition for comparison and classification purposes
The CIPW norm converts a rock's bulk chemical composition (major oxides from XRF) into a standard set of theoretical 'normative' minerals, allowing systematic classification and comparison of igneous rocks regardless of actual mineralogy or texture.
The CIPW normative calculation (Cross, Iddings, Pirsson, and Washington, 1902) provides a standardized way to calculate the theoretical mineral assemblage that would form if a magma crystallized under specific conditions. The calculation allocates the major oxide components (SiO2, Al2O3, CaO, MgO, FeO, Na2O, K2O, etc.) to normative minerals in a prescribed sequence. Normative minerals like Q (quartz), Or (orthoclase), Ab (albite), An (anorthite), Di (diopside), and Hy (hypersthene) are computed. The presence of normative Q indicates silica oversaturation; normative Ne (nepheline) indicates undersaturation. Used in the TAS and QAPF classification schemes.
Question 5: What are eclogites, and in what tectonic setting do they typically form?
- High-pressure, high-temperature metamorphic rocks composed of omphacite (Na-Al pyroxene) and pyrope-rich garnet, formed by deep subduction of crustal material (Correct answer)
- Low-grade metamorphic rocks formed in blueschist facies conditions at mid-ocean ridges
- Contact metamorphic rocks formed by magmatic intrusion into carbonate country rock
- Hydrothermal alteration products of basalt found at mid-ocean ridge hydrothermal vents
Correct answer: High-pressure, high-temperature metamorphic rocks composed of omphacite (Na-Al pyroxene) and pyrope-rich garnet, formed by deep subduction of crustal material
Eclogites are high-pressure metamorphic rocks characterized by the assemblage omphacite + pyrope garnet, formed when oceanic or continental crust is subducted to depths of 50–150 km (or deeper in ultra-high pressure terranes) where pressure exceeds the stability of plagioclase.
Eclogites represent the high-pressure end-member of crustal metamorphism, typically forming at pressures > 1.5–2.0 GPa (depths > 50–70 km) and temperatures of 450–900°C. The diagnostic assemblage is omphacite (Na,Al-bearing clinopyroxene) + pyrope-almandine garnet, ± kyanite, phengite, rutile, and coesite (high-P SiO2 polymorph). They form primarily in subduction zones where oceanic crust (eclogite facies blueschist → eclogite transition) or continental crust (ultra-high pressure, UHP, terranes) is subducted to great depths. Fragments of eclogite are found in diamond-bearing kimberlites and in exhumed metamorphic complexes.
Question 6: What is the difference between a concordant and a discordant igneous intrusion, and give examples of each?
- Concordant intrusions are felsic; discordant intrusions are mafic, regardless of their relationship to country rock structure
- Concordant intrusions (sills, laccoliths) are emplaced parallel to existing host rock layering; discordant intrusions (dikes, stocks, batholiths) cut across existing structures (Correct answer)
- Concordant intrusions cool faster because they have greater surface area contact with country rock; discordant intrusions cool slowly
- Concordant intrusions form in compression; discordant intrusions form in extensional tectonic settings
Correct answer: Concordant intrusions (sills, laccoliths) are emplaced parallel to existing host rock layering; discordant intrusions (dikes, stocks, batholiths) cut across existing structures
Concordant intrusions are parallel to the layering or foliation of the host rock (sills follow bedding; laccoliths dome up but remain parallel); discordant intrusions crosscut host rock structures (dikes cut across bedding; stocks and batholiths are large crosscutting plutons).
The geometry of magmatic intrusions relative to host rock structures is a fundamental classification in igneous geology. Concordant intrusions include: sills (tabular, parallel to bedding, horizontal to sub-horizontal), laccoliths (mushroom-shaped, parallel beneath an arched roof), and phacoliths (concordant in fold hinges). Discordant intrusions include: dikes (tabular, crosscut bedding), volcanic necks (cylindrical conduits), stocks (<100 km2 at surface), and batholiths (>100 km2). The principle of cross-cutting relationships (discordant intrusions are younger than the rocks they cut) is a key relative dating tool.
ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers 2
What is the Bowen's Reaction Series, and what does it predict about the order of mineral crystallization from a mafic magma?