A mineral is a substance with a specific chemical composition that occurs naturally in rocks. Min­erals are inorganic; being produced by the physi­cal forces of the Earth rather than by living organ­isms, and are usually in the form of crystals. Their importance lies in the fact that they are the basic units from which rocks are composed - every rock can be considered as an accumulation of minerals.

Most rocks consist of at least two different min­erals, and may contain five or six. The proportions of the constituent minerals vary - both among dif­ferent general types of rocks and among samples of the same type of rock. For this reason geologists do not classify rocks principally according to their compositions but by the way in which they were formed.

There are three broad categories of rocks: igneous, sedimentary and metamorphic.

• Igneous rocks are formed when molten magma cools and solidifies.

• Sedimentary rocks originate from deposits of material worn away from pre-existing rocks.

• Metamorphic rocks are formed by the alteration of pre-existing rocks by great heat or pressure.

On the Earth's surface sedimentary rocks are the most common, covering about 75 per cent of the total land area. Below the surface, however, they are much rarer, making up only about 5 per cent of the uppermost 16km of the crust; igneous and metamorphic rocks together account for the other 95 per cent.

Rock-forming Minerals

The proportions of minerals in the crust reflect the relative abundance of elements in the Earth. Oxygen is the most abundant element; hence many common minerals are in the form of oxides - compounds of oxygen and another element. Sili­con is the next most abundant element and so the mineral silica (the compound of silicon and oxygen) is very common.

Most of the rock-forming, or essential, minerals are silicates, in which silica is combined with one or more of the other common elements, such as aluminum, calcium, iron, magnesium, potassium or sodium. The essential minerals include feld­spars (e.g. orthoclase), micas (e.g. muscovite), amphiboles (e.g. hornblende), pyroxenes (e.g. augite), olivines (e.g. chrysolite) and quartz. Of these quartz is unusual in that it consists of only silicon and oxygen. Moreover, it is found only in rocks that have a far greater amount of silicon and oxygen than other elements, so that after these other elements have combined with silicon and oxygen to form complex silicate minerals there remains an excess of silica (in the form of quartz).

Crystals

Minerals are formed as crystals, the shape of which depends on the arrangement of their con­stituent atoms. But it is only rarely that a mineral's true crystalline shape can be seen by normal methods, although it is nearly always visible using X-ray crystallography.

A mineral can usually be identified once its crystal structure has been deter­mined.

When a molten mass cools and solidifies, the first minerals to crystallize out may form good crystal shapes, with straight edges and well-defined angles, because they have room to grow unimpeded in the surrounding fluid. The minerals that crystallize later fill the spaces left between the crystals already formed and so tend to be distorted.

The best-shaped crystals are usually those that grow early in the formation of an igneous rock, when there is still sufficient space in the fluid magma to enable their full development. Good crystal shapes may also form in metamorphic rocks but often the crystals grow under such high pressures that they become distorted.

In general, sedimentary rocks tend not to have good crystal shapes because the rocks are formed by the com­pression and solidification of fragments from other rocks and new minerals develop in the small spaces between these fragments. Sandstones - one of the most common types of sedimentary rock - consist mainly of quartz grains cemented together by calcite (calcium carbonate) or by oxides of iron. These cementing agents, which are dissolved in the groundwater that percolates through the sedi­ments, form tiny crystals between the grains, binding them together.

The minerals in extremely fine-grained sedimentary rocks consist of com­plex silicates called clay minerals; these are the weathered products of minerals such as feldspars and micas. Chemical sedimentary rocks - rock salt for example - are formed from substances that were originally dissolved in water and later pre­cipitated as a sediment that was then compressed and cemented together into rock. In these condi­tions well-defined crystals may form.

Ore Minerals

Minerals that are commercially valuable are known as ore minerals. Compared with rock-forming silicate minerals, ore minerals are rare, constituting only a very small proportion of the Earth's crust. In general, the ore minerals most commonly exploited for industrial purposes contain a high percentage of various metal ele­ments.

Iron oxide - which is probably most familiar as rust - is one of the most common ore minerals. It imparts a reddish color to many rocks, particu­larly sandstones formed from desert sands.

Hematite and magnetite are among the most abundant forms of naturally-occurring iron oxide.

Aluminum and tin are other valuable metals that are often found as oxide minerals, as bauxite and cassiterite respectively.

Some metals are found as compounds of the element sulfur (sulfides). Commercially important sulfide ores include pyrites (iron sulfide), cinnabar (mercury sulfide) and galena (lead sulfide).

Occasionally a metal may occur in its pure form, not chemically combined with other elements; the term "native" is applied to such pure minerals.