A selection of minerals. Credit: US Geological Survey.
Elements are won from minerals in different ways, according to their position on the periodic table. In the red group are reactive metals usually extracted by electrolysis. In the orange group, the elements frequently occur in ionic compounds, often combined with oxygen. These elements are often prepared by electrolysis. The third group (colored green) is commonly found as sulphides and the elements are obtained by roasting these and reducing the resultant oxides. Group four (colored yellow) are relatively unreactive elements, found free or as compounds that give the elements when they are heated to a certain temperature. The fifth group (light brown) are non-metals that occur free or as negative ions that can be converted to elements by electrolysis.
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.
Mineralogy is the investigation of minerals – naturally occurring inorganic substances found on Earth and elsewhere in the Solar System. Major subdivisions are: crystallography, which studies the composition and atomic arrangement of minerals; paramagnetic mineralogy, which deals with the associations and order of crystallization of minerals; descriptive mineralogy, concerned with the physical properties used in identification of minerals; and taxonomic mineralogy, the classification of minerals by chemical and crystal type.