Earth materials: minerals, rocks and their identification
This domain tests identification of minerals and rocks from diagnostic properties and textures, and the reasoning that links a material to its origin. Expect short identification tasks plus justification, not memorised lists.
Minerals
- A mineral is a naturally occurring, inorganic, crystalline solid with a definite composition or limited solid solution.
- Silicates dominate: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (micas, clays), frameworks (quartz, feldspars).
- Diagnostics: hardness (Mohs), cleavage, lustre, colour and streak, effervescence, magnetism.
- Carbonates, sulfates, sulfides, oxides and halides complete the common groups.
Rocks
- Igneous: texture (phaneritic, aphanitic, porphyritic, vesicular) plus composition (felsic to ultramafic). Granite, andesite, basalt, peridotite.
- Sedimentary: clastic rocks described by grain size, sorting, roundness, matrix and cement; chemical and biochemical rocks include limestone, chert, gypsum and halite. Carbonates are classified by texture (mudstone, wackestone, packstone, grainstone, boundstone).
- Metamorphic: foliated (slate, schist, gneiss) versus non-foliated (marble, quartzite, hornfels); grade and probable protolith matter.
Andalusian examples
Serrania de Ronda peridotites (olivine and pyroxene, often serpentinised); Macael marble (saccharoidal calcite); Sierra Morena granite and slate; Guadalquivir blue marls; Cabo de Gata andesite and dacite; Triassic gypsum.
Worked example
A dark green, dense, granular rock from the Serrania de Ronda contains olivine and pyroxene but no quartz. Identify it and its origin. Mafic-ultramafic mineralogy with a granular texture indicates peridotite, mantle material emplaced into the crust. If serpentine and slickensides appear, it is serpentinite, the hydrated equivalent.
Traps that cost marks
Confusing quartz and calcite: hardness 7 versus 3, and calcite effervesces in dilute HCl. Colour alone is unreliable for felsic versus mafic. Bedding is not foliation. Marble is distinguished from limestone by interlocking calcite and absence of fossils. Porphyritic texture is not porphyroblastic texture.
Key points
- Hardness, cleavage and effervescence identify common minerals quickly.
- Texture records cooling history or deformation, not just composition.
- Carbonates are named by depositional texture.
- Peridotite and serpentinite mark the Ronda mantle section.
- Marble is metamorphosed limestone; quartzite is metamorphosed sandstone.