Minerals, Rocks and the Rock Cycle: Identification, Classification and Economic Interest
This domain tests practical identification and classification: given a set of properties or a described hand sample, you must name the mineral or rock, explain its origin and place it in the rock cycle. It also covers economic use, which matters in an island context.
Minerals
A mineral is natural, inorganic, solid, with a definite chemical composition and an ordered crystalline structure. Diagnostic properties: hardness (Mohs: talc 1, calcite 3, quartz 7, corundum 9, diamond 10), cleavage versus fracture, streak, lustre, colour, density, magnetism and reaction with dilute HCl (calcite effervesces; dolomite only when powdered).
Silicate structures control properties: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (micas, clays), frameworks (quartz, feldspars).
Bowen's reaction series
Discontinuous branch: olivine → pyroxene → amphibole → biotite. Continuous branch: Ca-plagioclase → Na-plagioclase. Both converge on K-feldspar, muscovite and quartz, the minerals stable at the lowest temperatures.
Rock classification
- Igneous: plutonic/volcanic and felsic/intermediate/mafic/ultramafic. Granite, granodiorite, gabbro, basalt, rhyolite, andesite.
- Sedimentary: detrital (conglomerate, sandstone, mudstone), chemical (limestone, evaporites, chert) and organic (coal, oil source rocks).
- Metamorphic: foliated (slate, schist, gneiss) and non-foliated (marble, quartzite, hornfels).
Rock cycle and economic interest
The cycle links internal processes (magmatism, metamorphism, uplift) with external ones (weathering, erosion, transport, deposition, diagenesis). Economically important materials include limestone and marès as building stone, gypsum and salt from evaporites, clays for ceramics, aggregates, and groundwater hosted in karstic limestones.
Worked example
A coarse-grained rock with interlocking quartz, K-feldspar and muscovite and no foliation is a granite: coarse texture means slow cooling at depth (plutonic), and the quartz-feldspar-mica assemblage is felsic. If a cobble of this granite appears inside a conglomerate, the granite is older than the conglomerate, and the conglomerate records erosion of a granitic source area.
Traps that cost marks
- Confusing crystal habit with hardness.
- Forgetting that marble is metamorphosed limestone and quartzite is metamorphosed sandstone, not the reverse.
- Calling all limestones chemical when many are biogenic.
- Claiming clay is permeable because it holds water; it has high porosity but very low permeability.