Earth Materials: Minerals, Rocks and Their Identification
Minerals first
A mineral is naturally occurring, inorganic, solid, with a definite chemical composition (within limits) and an ordered internal structure. Anything failing one criterion is not a mineral (glass lacks order; coal is organic).
Key properties and what they tell you:
- Cleavage vs fracture: cleavage follows planes of weak bonding; fracture is irregular (quartz shows conchoidal fracture).
- Hardness (Mohs): talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10.
- Streak is more reliable than colour for metallic minerals such as hematite.
- Silicate structures control properties: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (mica, clay), frameworks (quartz, feldspar).
Rocks
- Igneous: intrusive (coarse, slow cooling) vs extrusive (fine, rapid cooling); felsic (high silica, pale, viscous) vs mafic (low silica, dark, fluid).
- Sedimentary: clastic (grain size sorting), chemical (evaporites, limestone) and biogenic.
- Metamorphic: foliated (slate, schist, gneiss) vs non-foliated (marble, quartzite); grade increases with heat and pressure.
Traps
- Identifying by colour alone - quartz, calcite and halite can all be pale.
- Confusing cleavage planes with crystal faces (faces grow; cleavage breaks).
- Assuming all dark rocks are mafic; grain size and texture matter.
- Forgetting that metamorphism is solid-state - melting produces igneous rock.
Worked example
A dark, fine-grained rock with no visible crystals, abundant olivine and pyroxene, and vesicles is basalt: extrusive, mafic, formed by rapid cooling of low-viscosity lava. Compare gabbro - same composition, coarse-grained, intrusive. Rule: composition from minerals, texture from cooling history.