Mineralogy
What the exam tests
Mineralogy covers the definition of a mineral, crystal habit, physical and optical properties, the silicate structure classes, and identification of the rock-forming minerals found in Kerala's granulites and beach sands.
Core rules
- A mineral is naturally occurring, inorganic, solid, with a definite chemical composition and an ordered atomic arrangement. Glass and opal lack the ordered lattice and are not minerals in the strict sense.
- Silicate classes by tetrahedral linkage: isolated (olivine), single chain (pyroxene, e.g. augite), double chain (amphibole, e.g. hornblende), sheet (biotite, muscovite, clay), framework (quartz, feldspar).
- Mohs scale: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Fingernail ≈2.5, copper coin ≈3, glass ≈5.5, steel nail ≈6.5.
- Cleavage follows planes of weak bonding; fracture is irregular. Specific gravity is a ratio and carries no unit.
Worked example
A mineral is colourless, hardness 3, shows three cleavages meeting at about 75° and 105°, and fizzes with dilute HCl. Step 1: effervescence identifies a carbonate. Step 2: hardness 3 fixes the carbonate as calcite rather than dolomite. Step 3: the rhombohedral angle (not 90°) excludes halite. Answer: calcite.
Traps
Saying quartz has cleavage - it shows conchoidal fracture. Confusing biotite's perfect basal cleavage with its colour. Giving specific gravity in g/cm³, which is density, not SG. Ignoring plagioclase striations and reporting every white feldspar as orthoclase.