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Fault
Essential mineral
The dominant mineral parts of a rock that are used to classify it. For example, quartz is an essential mineral in granite because it is one of the main parts of the rock.
Evaporite
A number of minerals are formed by the evaporation of seawater. They are collectively called evaporites, of which the most common are halite (salt, sodium chloride) and gypsum (calcium sulphate). They are very soft minerals (2 on Mohs’ scale of hardness) and occur as thick beds. They are very soluble minerals, only found in sedimentary rocks. (See also: Halide minerals.)
Extrusive rock, extrusion
An igneous rock that has solidified on the surface of the Earth.
The common word for this kind of rock is lava.
F
Facet
The cut and polished face of
a gemstone. Used in describing jewellery. It is not related to the cleavage planes of a mineral, but is imposed on the gemstone by the skill of the jeweller. (See also: Diamond.)
Facies
Physical, chemical, or biological variations in a sedimentary bed (for example, sandy facies, limestone facies). These differences arise because both clays and sand may be deposited
at the same time – clays offshore and sands close to the coast. A rock layer made from these deposits may therefore be shale (made
of clay) in one place, gradually changing to a sandstone nearby.
Fault
A break in the Earth’s rocks due to movements of the crust. Faults are of three kinds: normal, reversed and transcurrent.
 Evaporite – Evaporites form naturally in coastal lagoons and in inland basins. This is Nevada.
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