A selection of minerals.
Credit: U.S. Geological Survey.
A mineral is a naturally-occurring inorganic element or compound that has 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. A vein is a mineral formation of far greater extent in two dimensions than in the third. Sheetlike fissure veins occur where fissures formed in the rock become filled with mineral. Ladder veins formed in series of fractures in, for example, dikes. Saddle veins are lens-shaped, concave below and convex above. Veins that contain economically important ores are often termed lodes.
Rocks, such as granite or sandstone, are mixtures of minerals. Their compositions can vary widely and cannot be approximated by a mineral formula.