What is a mineral?
Back to the Mineral Identification
A mineral is a naturally occurring compound
or element found in the Earth's crust. Unlike rocks which minerals
build, a mineral has the same chemical composition throughout.
Unlike many of the compounds that we see in the laboratory,
the formula of minerals is not quite so predictable. The formula
can have a range of values and still be recognizable as a particular mineral.
For example, the mineral Olivine, which is a pale green mineral can have
a formula ranging from Mg2SiO4 to Fe2SiO4.
Minerals form in and on the Earth in a variety of
ways. The processes of mineral formation may be classified as follows:
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Igneous Mineral Formation:
Igneous minerals are crystallized from magma (molten
rock) at temperatures generally between 600oC and 1200oC,
and from the surface to depths of 30 kilometres or more.
Some minerals are formed by crystallizing under surface
conditions following chemical reactions between other minerals and conditions
on the surface of the Earth.
These are formed in one of three ways:
1. By the evaporation of water,
forming evaporative minerals such as halite (NaCl).
2. By being precipitated from water due to changes in
the chemical conditions eg chert (silica), iron oxides, carbonates.
3. By deposition by organisms as shells or bones eg aragonite
Metamorphic minerals occur as the result of recrystallisation
and reaction within existing rocks which produces new minerals in response
to changes in pressure and heat
Structure of Minerals
Most minerals occur as crystals when conditions are
suitable. In most cases these crystals are too small and too spread
out to be of any value in identifying a mineral. In these cases special
laboratory tests are necessary. In larger samples, the shape of a
crystal can be important in helping to identify the mineral. We will
not delve too far into the science of Crystallography here. Instead,
we will only refer to the common shapes that help us in the field.
The most useful structural help is the FORM
of the mineral. This refers to the shape of the lump that you dig