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a mineral from the group of natural fluorides, with chemical composition Na2NaAlF6.
In cryolite’s structure, the Al and one-third of the Na are located in the center of the AlF6 and NaF6 octahedrons, while two-thirds of the Na is in the center of the NaF12 polyhedrons. The mineral crystallizes in the monoclinic system; cubic crystals are rare. Ordinarily, colorless, white, or gray crystal aggregates with a vitreous luster form. Cryolite has a hardness of 2.5–3.0 on the mineralogical scale and a density of 2,960–2,970 kg/m3. It is found in metasomatically displaced pegmatites and is formed from fluoride-enriched hot water solutions associated with alkaline granites. Industrial deposits are rare (Ivigtut in western Greenland).
Cryolite is used extensively in aluminum metallurgy; it is also used to obtain enamels and for other purposes. Most of the cryolite used in industry is obtained synthetically [by the interaction of the sulfates of Al and NaF; by neutralization of gaseous fluosilicic acid (H2SiF6) with aluminum hydroxide and NaOH].
The mineral is called cryolite because of its resemblance in luster and refractive index to ice. Some scientists use the term “cryolite” for all varieties of ice (ice, snow, hail); but others, only for ice, as a monomineral rock.