Chemistry:

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.

Formation:

Minerals form in and on the Earth in a variety of ways.  The processes of mineral formation may be classified as follows:

IGNEOUS
WEATHERING
SEDIMENTARY
METAMORPHIC
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Igneous Mineral Formation:

Igneous minerals are crystallized from magma (molten rock) at temperatures generally between 600^oC and 1200^oC, and from the surface to depths of    30 kilometres or more. 

Minerals Formed by Weathering:

Some minerals are formed by crystallizing under surface conditions following chemical reactions between other minerals and conditions on the surface of the Earth. 

Sedimentary Minerals:

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 (CaCO₃) 

Metamorphic Minerals:

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 or HABIT of the mineral.  This refers to the shape of the lump that you dig up.