A selection of minerals.
Credit: U.S. Geological Survey.
A mineral is a naturally-occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal morphology, and physical properties. Minerals are the fundamental units from which most rocks are made. of the 3,000 or so minerals known, fewer than 100 are common.
Minerals may be identified by their color (though this often varies because of impurities), hardness (measured on Moh's scale), luster, specific gravity, crystal forms and cleavage; or by chemical analysis and X-ray diffraction. Minerals are generally classified by their anions (negative ions) – in order of increasing complexity: elements, sulfides, oxides, halides, carbonates, nitrates, sulfates, phosphates, and silicates. Others are classed with those which they resemble chemically and structurally, e.g., arsenates with phosphates. A newer system classifies minerals by their topological structure.
Minerals sometimes occur in veins. Rocks, such as granite or sandstone, are mixtures of minerals. Their compositions can vary widely and cannot be approximated by a mineral formula.