Cryolite

Cryolite is a rare mineral composed of sodium, aluminium, and fluorine, with the formula Na3AlF6. It was first described in 1798 by the Danish veterinarian and physician Peter Christian Abildgaard, who examined rock samples that local Inuit had been using to clean animal hides. The actual source of the ore was identified in 1806 by explorer Karl Ludwig Giesecke, who located the deposit at Ivittuut (historically spelled Ivigtut) and the nearby Arsuk Fjord on the southwest coast of Greenland. The mineral’s name comes from Greek words meaning “frost” and “stone.” The Pennsylvania Salt Manufacturing Company processed large quantities of cryolite to produce caustic soda and fluorine compounds, including hydrofluoric acid, at its plants in Natrona and Cornwells Heights, Pennsylvania, during the nineteenth and twentieth centuries. Cryolite was historically used as an ore of aluminium, but its main industrial role became that of a flux in the electrolytic smelting of bauxite, the aluminium-rich oxide ore. Pure cryolite melts at 1012 °C and can dissolve aluminium oxides, allowing aluminium extraction via electrolysis at much lower temperatures than melting the oxides alone. Because natural cryolite is now too scarce for this purpose, synthetic sodium aluminium fluoride is manufactured from the common mineral fluorite. In 1940, before the United States entered World War II, it became involved in protecting the Ivittuut mine from falling under Nazi German control. Commercial mining at Ivittuut ceased in 1987. Small deposits have also been reported in Spain, near Pikes Peak in Colorado, at Francon Quarry near Montreal in Quebec, and in Miass, Russia. Cryolite is used as an insecticide and pesticide, and it gives fireworks a yellow color. In glassmaking, it serves as a powerful opaliser. Physically, cryolite occurs as glassy, colorless, white-reddish to gray-black prismatic monoclinic crystals with a Mohs hardness of 2.5 to 3 and a specific gravity around 2.95 to 3.0. It is translucent to transparent and has a very low refractive index near 1.34, so it becomes nearly invisible when placed in water.