GeologyRocks

Amethyst

Variant of quartz, purple in colour.

Amphibole

A common rock forming (igneous or metamorphic) group of minerals with complex chemical compositions, but mostly ferro-magnesian silicates. They are composed of double chain SiO₄ tetrahedra. Amphiboles show two cleavage planes at around 120 degrees to each other.

Andalusite

A polymorph whose other two minerals are kyanite and sillimanite. A particularly notable variety of andalusite is chiastolite, which has crosses of carbon in it.

Apatite

Phosphate mineral, which can be in three forms depending on its chemistry. Forms are Fluorapatite (fluorine-rich), Chlorapatite (chlorine-rich and Hydroxylapatite (rich in hyrdroxides). It is also what most teeth of mammals are made of!

Aragonite

A polymorph of calcite. Produced by many organisms as a shell, test or other hard part. Unstable at the Earth's surface and is very rare in ancient sediments.

Calcite

Calcite is a mineral composed of Calcium Carbonate (CaCO₃). It exists as two forms, low and high Magnesium (Mg). Only low Mg is stable at the Earth's surface. Calcium Carbonate can also be found as aragonite, which is a polymorph of calcite. Calcite crystals may also show double refraction.

Chert

Microcrystalline silica (SiO₂). Flint is the best known example.

Citrine

Variety of quartz. Yellow-orange in colour.

Cleavage

The distinct planes of weakness that a mineral breaks along. The picture below shows clear cleavage lines.

Garnet

A cubic mineral of the general form of:

A₃²⁺B₂³⁺Si₃O₁₂

A can be magnesium, iron, manganese or calcium, B is iron or aluminium (occasionally chromium). Commonly found in metamorphic and occasionally igneous rocks.