Mineralogy
Mineralogy is the systematic study of minerals — naturally occurring, inorganic, crystalline solids of definite chemical composition. About 5,800 mineral species are recognised by the International Mineralogical Association (IMA), but only a few dozen — chiefly silicates, oxides and carbonates — make up the vast bulk of the Earth's crust.
A naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic arrangement (crystal structure). The five tests: (1) naturally formed, (2) inorganic, (3) solid, (4) ordered internal structure, (5) definite (but possibly variable) chemical composition.
Crystal systems
All crystalline minerals belong to one of seven crystal systems, classified by their symmetry and unit-cell shape:
| System | Axes | Angles | Example |
|---|---|---|---|
| Cubic (isometric) | a = b = c | All 90° | Halite, pyrite, diamond |
| Tetragonal | a = b ≠ c | All 90° | Zircon, rutile |
| Orthorhombic | a ≠ b ≠ c | All 90° | Olivine, topaz |
| Hexagonal | a = b ≠ c | 90°, 90°, 120° | Quartz, beryl |
| Trigonal (rhombohedral) | a = b = c | All equal, ≠ 90° | Calcite, hematite |
| Monoclinic | a ≠ b ≠ c | Two 90°, one ≠ 90° | Gypsum, orthoclase, micas |
| Triclinic | a ≠ b ≠ c | None 90° | Plagioclase, kyanite |
The internal atomic ordering gives rise to cleavage, fracture, and external crystal morphology.
Physical properties
Mineral identification in the field rests on simple physical tests:
- Mohs hardness scale (1812): talc 1 → diamond 10. Each mineral scratches any softer one. Useful intermediates: fingernail (2.5), copper coin (3.5), steel knife (5.5), quartz (7).
- Cleavage: tendency to break along specific crystallographic planes (mica — one direction; halite — three at 90°; calcite — three not at 90°; fluorite — four).
- Fracture: irregular breakage (conchoidal in quartz, hackly in metals).
- Lustre: metallic, vitreous, pearly, silky, adamantine, dull.
- Streak: colour of powdered mineral on a porcelain plate; more diagnostic than external colour.
- Specific gravity: ratio of density to that of water; ranges from ~2.0 (sulphates) to >20 (native osmium).
- Mohs scale: 1-Talc, 2-Gypsum, 3-Calcite, 4-Fluorite, 5-Apatite, 6-Orthoclase, 7-Quartz, 8-Topaz, 9-Corundum, 10-Diamond.
- Quartz is the most common single mineral in the upper crust; the silicate group is overall ~92 % of crustal volume.
- Isomorphism: minerals with the same structure but different chemistry (e.g., olivine series Fo–Fa).
- Polymorphism: same chemistry, different structures (graphite vs diamond; calcite vs aragonite).
Mineral classes (Dana / Strunz)
Minerals are grouped chemically by their dominant anion or anionic complex:
- Native elements: gold, silver, copper, diamond, graphite, sulfur.
- Sulphides: galena (PbS), pyrite (FeS₂), chalcopyrite (CuFeS₂), sphalerite (ZnS). Major sources of metal ores.
- Oxides and hydroxides: hematite (Fe₂O₃), magnetite (Fe₃O₄), corundum (Al₂O₃), rutile (TiO₂), bauxite (Al(OH)₃-rich rock).
- Halides: halite (NaCl), fluorite (CaF₂), sylvite (KCl).
- Carbonates: calcite (CaCO₃), dolomite (CaMg(CO₃)₂), aragonite, siderite.
- Sulphates: gypsum (CaSO₄·2H₂O), anhydrite, barite (BaSO₄).
- Phosphates: apatite (Ca₅(PO₄)₃(F, Cl, OH)).
- Silicates — by far the largest class, structured around the (SiO₄)⁴⁻ tetrahedron.
Silicate classification
Silicates are subdivided by polymerisation of the SiO₄ tetrahedra:
| Subclass | Structure | Si : O | Examples |
|---|---|---|---|
| Nesosilicates | Isolated tetrahedra | 1 : 4 | Olivine, garnet, zircon |
| Sorosilicates | Paired tetrahedra | 2 : 7 | Epidote |
| Cyclosilicates | Rings | 1 : 3 | Beryl, tourmaline |
| Inosilicates (single chain) | Single chains | 1 : 3 | Pyroxenes (augite) |
| Inosilicates (double chain) | Double chains | 4 : 11 | Amphiboles (hornblende) |
| Phyllosilicates | Sheets | 2 : 5 | Micas, clays |
| Tectosilicates | 3-D framework | 1 : 2 | Quartz, feldspars |
About 92 % of the crust by volume is silicate; feldspars alone make up over 50 %.
Optical properties
Under a polarising microscope minerals are identified by:
- Colour in plane-polarised light (PPL) and pleochroism (colour change with stage rotation).
- Relief (apparent contrast against mounting medium; reflects refractive index).
- Birefringence = n_max − n_min, producing interference colours under crossed polars (XPL).
- Extinction angle between cleavage and the dark position.
- Twinning patterns (especially diagnostic in plagioclase — albite twinning).
- Isotropic (cubic) minerals stay dark under crossed polars; anisotropic minerals show interference colours.
Two simple field rules:
- If a soft, transparent crystal effervesces in dilute HCl, it is almost certainly calcite.
- A heavy black mineral that strongly attracts a magnet is magnetite. Hematite is heavy and reddish-brown but only weakly magnetic.
Economic and gem minerals
Many minerals have economic significance:
- Ore minerals: hematite, magnetite, bauxite, sphalerite, galena, chalcopyrite, chromite.
- Industrial minerals: gypsum (plaster), halite (chemical industry), apatite (fertilisers), kaolinite (ceramics), talc.
- Gems: diamond (C), corundum (ruby/sapphire, Al₂O₃), beryl (emerald, aquamarine), topaz, garnet, tourmaline, opal (amorphous SiO₂·nH₂O — not strictly a mineral by some definitions).
Gem value is governed by the four Cs — carat, clarity, colour, cut.
Mineral genesis
Minerals form by:
- Crystallisation from a melt (igneous: feldspars, micas, pyroxenes; Bowen's reaction series).
- Precipitation from aqueous solution (evaporites: halite, gypsum; carbonates).
- Solid-state reaction in response to changes in pressure and temperature (metamorphic: garnet, staurolite, kyanite, sillimanite).
- Hydrothermal deposition from hot mineralising fluids (quartz veins, sulphide deposits).
- Biogenic precipitation (mollusc shells = aragonite/calcite; coccolith chalk).
Understanding the conditions of formation — the paragenesis — places minerals in their geological context and is the bridge from mineralogy to petrology, geochemistry and economic geology.