Minerals, rocks and geological resources
Domain 6 - Minerals, rocks and geological resources. Typical questions ask you to identify a mineral or rock from descriptive properties, classify it, then discuss its economic use and environmental cost.
Minerals
- Definition: naturally occurring, inorganic, crystalline solid with a definite composition. Glass and obsidian fail the test.
- Identification: hardness (Mohs 1 talc to 10 diamond), cleavage versus fracture, lustre, streak, colour, specific gravity, crystal habit, reaction with dilute HCl.
- Silicates dominate the crust: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (micas, clays), frameworks (quartz, feldspars). Non-silicates: carbonates (calcite, dolomite), sulphides (pyrite, galena), oxides (hematite, magnetite), sulphates (gypsum), halides (halite).
Rocks
- Igneous: classified by texture (phaneritic, aphanitic, porphyritic, glassy, vesicular) and composition (felsic to ultramafic).
- Sedimentary: clastic (conglomerate, sandstone, shale) by grain size; chemical and biochemical (limestone, dolostone, evaporites, chert); organic (coal).
- Metamorphic: foliated (slate, schist, gneiss) versus non-foliated (marble, quartzite, hornfels).
Resources and reserves
- Ore = mineral + gangue; grade and cut-off grade decide viability. A resource is a concentration; a reserve is the part that is economically recoverable with current technology and prices.
- Spanish examples: the Iberian Pyrite Belt (Riotinto, Tharsis - Cu, Zn, Pb, S), Almadén mercury, potash in Catalonia, lithium projects in Extremadura, and aggregates and ornamental stone (Marble of Macael).
- Critical raw materials (EU list) drive recycling and substitution policy.
- Environmental impacts: acid mine drainage from sulphide oxidation, tailings dam failures, subsidence, dust, land take, and water use. Mitigation: neutralisation, liners, revegetation, circular economy.
Worked example
A mineral scratches a copper coin but not glass, shows perfect rhombohedral cleavage and effervesces in dilute HCl. Hardness is therefore about 3 (calcite), consistent with the cleavage and the carbonate reaction; it is calcite, used in cement and lime, and its dissolution also drives karst.
Traps
- Confusing cleavage with crystal habit or with fracture.
- Quoting density in g/L instead of g/cm3, or mass as weight.
- Calling any concentration an ore body - profitability matters.
- Ignoring gangue when calculating ore grade.