common halide mineral, calcium fluoride (CaF₂); the principal fluorine mineral. It is usually quite pure, but as much as 20 percent yttrium or cerium may replace calcium. Fluorite occurs most commonly as a glassy, many-hued vein mineral and is often associated with lead and silver ores; it also occurs in cavities, in sedimentary rocks, in pegmatites, and in hot-springs areas. It is widespread in China, South Africa, Mongolia, France, Mexico, Russia, and the central United States. Fluorite is used as a flux in the manufacture of open-hearth steel, of aluminum fluoride, of artificial cryolite, and of aluminum. It is used in opalescent glass, in iron and steel enamelware, in the production of hydrofluoric acid, in the refining of lead and antimony, and in the manufacture of high-octane fuels (as a catalyst). Because of its low index of refraction and low dispersion, clear, colourless fluorite of optical quality is used for apochromatic lenses. At one time blue john, a variety from Derbyshire, Eng., was widely used in ornamental vases and other objects. For detailed physical properties, see halide mineral (table).