Mineralogy
Mineralogy tests identification of minerals by physical properties, crystal systems and symmetry, silicate structures, and simple optical behaviour. Karnataka's ore and industrial minerals make local examples useful.
Core rules
- A mineral is naturally occurring, inorganic, crystalline and has a definite chemical composition (or limited solid solution).
- Diagnostic properties include hardness (Mohs scale), cleavage, fracture, lustre, streak, colour, specific gravity and magnetism.
- Cleavage is a repeated planar break controlled by weak bonds; fracture is irregular breakage.
- Silicates are classified by Si:O ratio and tetrahedral linkage: isolated, chain, sheet and framework structures.
- In thin section, colour, relief, cleavage, extinction and birefringence help identify rock-forming minerals.
Worked example
A specimen is soft enough to be scratched by a copper coin, shows three cleavages at oblique angles, and effervesces with dilute HCl. Identify it. Reasoning: hardness about 3 rules out quartz (H=7); rhombohedral cleavage and acid reaction identify calcite. Quartz would instead show conchoidal fracture, no cleavage, and no reaction.
Traps
- Colour is unreliable; streak and hardness are more diagnostic.
- Crystal form is not the same as cleavage.
- "Shiny" is not a lustre term; use metallic, vitreous, pearly, silky or earthy.
- Do not identify a mineral from a single property.
Karnataka link
Gold occurs in quartz veins in schist belts; magnetite and haematite are iron ores; chromite is dark, heavy and weakly magnetic.