Geology: Rocks, Minerals and the Rock Cycle
Minerals and identification
A mineral is naturally occurring, inorganic, with a definite composition and ordered atomic structure. Most rock-formers are silicates built on the SiO4 tetrahedron. Memorise Mohs in order: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10. Field identification uses hardness, cleavage, lustre, streak, colour and specific gravity. Quartz: hardness 7, conchoidal fracture, no cleavage, vitreous. Calcite: hardness 3, rhombohedral cleavage, effervesces in dilute HCl. In Deccan basalt, augite and olivine are the dark ferromagnesian minerals; plagioclase feldspar is the pale component.
The three rock families
Igneous rocks crystallise from magma - intrusive (coarse: granite, gabbro dykes) or extrusive (fine: basalt, andesite, volcanic tuff). Sedimentary rocks form by deposition and cementation: sandstone, shale, limestone. Laterite is a residual weathering crust on the Konkan plateaus, not an igneous rock. Metamorphic rocks form under heat and pressure: granite → gneiss, shale → slate → schist, limestone → marble, sandstone → quartzite; gneiss and charnockite occur in eastern Vidarbha. The rock cycle links all three through weathering, burial, metamorphism and melting.
Worked example
A grey, glassy grain scratches glass, has no flat cleavage faces and gives a white streak. Scratching glass means hardness above 5.5; no cleavage with conchoidal fracture and vitreous lustre identifies quartz. Calcite would not scratch glass and would show cleavage.
Traps
Basalt is extrusive, not intrusive. Laterite is residual/sedimentary, not volcanic. Diamond and graphite are both carbon - hardness depends on structure, not composition. Never identify by colour alone.