Mineralogy, Petrology, and Geochemistry
What to know
Mineral identification uses crystal system, cleavage, hardness, specific gravity, luster, and optical properties. Silicate structures run from isolated tetrahedra (olivine) through single and double chains (pyroxene, amphibole) to sheets (micas, clays) and frameworks (quartz, feldspars); this progression tracks rising Si:O and generally lower density, and controls cleavage (micas = one perfect sheet cleavage; pyroxene = two at ~90°; amphibole = two at ~60°/120°).
Igneous rocks are classified by texture (cooling history) and composition; the IUGS QAPF scheme plots modal quartz, alkali feldspar, and plagioclase. Bowen's reaction series describes crystallization order and, reversed, partial melting: mantle peridotite melts to basalt, and fractional crystallization drives residual liquids toward andesite and rhyolite while Mg, Fe, Ca, and Ni fall and Si, K, and Na rise. Metamorphic rocks are classified by facies - mineral assemblages recording temperature and pressure - not by texture alone.
Geochemistry separates compatible elements (Ni, Cr, Mg) from incompatible elements (Rb, Ba, U, Th, Nb) using partition coefficients. Radiogenic isotopes trace source and age; stable isotopes (O, H, C, S) trace processes such as temperature, fluid source, and evaporation.
Traps
Confusing polymorphs (quartz/coesite/stishovite; calcite/aragonite; kyanite/sillimanite/andalusite); calling a glassy rock a mineralogic class; assuming high-grade metamorphism always means high pressure; mistaking a cumulate for a fractionated liquid composition.
Rule of thumb
Quartz + K-feldspar + muscovite = felsic; olivine + pyroxene with little feldspar = ultramafic. Use mineral assemblage first, texture second.