Mineralogy and Petrology
Domain focus
Mineralogy and Petrology tests mineral identification, crystal chemistry, silicate structures, igneous differentiation, sedimentary classification, and metamorphic facies. The exam expects you to link a mineral assemblage or texture to a process, not just memorize rock names.
Core rules and concepts
- Silicate structures control properties: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (mica, clay), frameworks (quartz, feldspar). Cleavage reflects weak bonds; hardness reflects bond strength.
- Bowen's reaction series explains early mafic crystallization and later felsic crystallization. Plagioclase shifts from Ca-rich to Na-rich as cooling proceeds.
- Igneous rocks are classified by texture and mineralogy; sedimentary rocks by grain size/composition; metamorphic rocks by fabric and index minerals.
- Metamorphic facies are pressure-temperature assemblages. A pelitic schist with garnet + staurolite + muscovite + quartz indicates amphibolite-facies conditions, not greenschist.
Worked example
A porphyritic rock contains large olivine and Ca-plagioclase phenocrysts in a fine-grained groundmass. Early olivine and Ca-plagioclase removal from a mafic melt leaves residual liquid enriched in Si, Na, and K. If that liquid crystallizes, it can form a more felsic rock. The reasoning is fractional crystallization plus Bowen's series: mafic phenocrysts record early crystallization, while the groundmass records the evolved residual melt.
Traps
Do not identify a rock from color alone: gabbro and basalt share mineralogy but differ in texture. Do not confuse cleavage with crystal form or fracture. In metamorphic rocks, identify the protolith and the assemblage before naming the facies. A single index mineral is weaker evidence than a consistent assemblage.