Earth Materials and Mineralogy
What this domain tests
The Earth Materials and Mineralogy domain examines whether you can relate a mineral's internal structure to its physical and optical properties, identify specimens in hand specimen and thin section, and use silicate classification and Bowen's reaction series to explain mineral associations. GCE questions often ask you to name a mineral from a property table, compare cleavage and crystal form, or interpret an optical description.
Core rules and concepts
- A mineral is naturally occurring, inorganic, crystalline and has a definite (though variable) chemical composition; a rock is an aggregate of minerals.
- The silicate tetrahedron (SiO4)4- is the basic building block. Structure controls properties: isolated tetrahedra (olivine), single chains (pyroxene), double chains (amphibole), sheets (mica, clay), frameworks (quartz, feldspar). The Si:O ratio rises from 1:4 in olivine to 1:2 in quartz.
- Mohs hardness: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Hardness is a scratch resistance, not a measure of strength.
- Cleavage is controlled by planes of weakness in the lattice. Halite has three cleavages at 90°, galena three at 90°, calcite three rhombohedral, mica one perfect basal, amphibole two at about 60°/120°, pyroxene two at about 87°/93°.
- In thin section: isotropic minerals (e.g., garnet, fluorite) are dark under crossed polars; anisotropic minerals show interference colours. Plagioclase shows albite twinning; microcline shows tartan twinning; biotite and hornblende are strongly pleochroic.
Worked example
A dark green mineral has hardness 5.5-6, two cleavages meeting at about 60°, SG 3.2, and is pleochroic from green to brown. Candidate A is augite: two cleavages at about 90°, SG 3.3, weakly pleochroic, colourless to pale green in thin section. Candidate B is hornblende: two cleavages at 60°/120°, SG 3.2, strong green-brown pleochroism. The cleavage angle and pleochroism point to hornblende, an amphibole. The double-chain structure accounts for the 60° cleavage and the OH group contributes to the strong colour.
Traps that cost marks
Confusing cleavage with crystal faces; quartz has no cleavage and breaks conchoidally. Naming a mineral from colour alone - hematite may be red or black. Forgetting that calcite effervesces in dilute HCl while dolomite reacts only when powdered. Giving the number of cleavages without the angle. Confusing hardness with streak.