Minerals and rocks: identification, classification and uses
This domain is identification-led: you must recognise minerals from their properties, classify rocks from texture and composition, and connect both to economic uses. The PAU rewards precise property vocabulary and correct classification hierarchies.
Minerals
A mineral is natural, inorganic, solid, crystalline, with a definite chemical composition. Silicates dominate and are classified by tetrahedron arrangement: nesosilicates (olivine, garnet, kyanite), sorosilicates (epidote), cyclosilicates (beryl), inosilicates (pyroxene single chain, amphibole double chain), phyllosilicates (mica, clay) and tectosilicates (quartz, feldspar). Key properties: Mohs hardness from talc to diamond, cleavage versus fracture, streak, lustre, colour, density, magnetism and effervescence with HCl (calcite). Polymorphs such as calcite–aragonite or kyanite–sillimanite record pressure conditions.
Rock classification
- Igneous: plutonic (coarse; granite, diorite, gabbro) versus volcanic (fine; rhyolite, andesite, basalt); felsic to mafic by silica content and colour index.
- Sedimentary: clastic (conglomerate, sandstone, lutite), chemical (rock salt, gypsum, chert) and biochemical (limestone, coal).
- Metamorphic: foliated (slate, schist, gneiss, migmatite) and non-foliated (marble, quartzite, hornfels).
Uses
Limestone and clay for cement, gypsum for plaster, clays for ceramics, granite and marble as ornamental stone, evaporites for salt and potash, silica sand for glass, and sulphide ores such as those of the Iberian Pyrite Belt.
Worked example
A coarse, pale rock shows interlocking grains of quartz, pink potassium feldspar, white plagioclase and black biotite; it scratches glass and does not react with acid. Reasoning: coarse interlocking crystals mean plutonic; quartz plus alkali feldspar means felsic; the absence of foliation excludes gneiss; the acid test excludes marble; the rock is granite.
Traps that cost marks
- Confusing cleavage with crystal faces; quartz has fracture but no cleavage.
- Calling a rock a mineral, as in "quartzite mineral".
- Classifying by colour alone: dark colour may come from heavy minerals rather than a mafic bulk composition.
- Forgetting that marble effervesces in acid and quartzite does not.