Cryolite
Rare mineral used in aluminium smelting and pest control.
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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.
Source locations
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.
Physical properties
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.
Lore & Background
The name is derived from the Greek words cryos (frost) and lithos (stone). The Pennsylvania Salt Manufacturing Company used large amounts of cryolite to make caustic soda and fluorine compounds, including hydrofluoric acid at its Natrona, Pennsylvania, works, and at its integrated chemical plant in Cornwells Heights, Pennsylvania, during the 19th and 20th centuries.
It was historically used as an ore of aluminium and later in the electrolytic processing of the aluminium-rich oxide ore bauxite. The difficulty of separating aluminium from oxygen in the oxide ores was overcome by the use of cryolite as a flux to dissolve the oxide mineral(s). According to economist Arindam Banerjee, exploitation of cryolite in Greenland contributed nearly 54 billion euros to Danish economy, though this claim has been strongly disputed.
Reader's Guide
Cryolite's primary historical significance lies in its role as a flux in the Hall–Héroult process for refining aluminium. Although natural cryolite is now too rare for this purpose, synthetic sodium aluminium fluoride produced from fluorite has replaced it.
The mineral also found use as an insecticide, pesticide, a yellow colorant in fireworks, and as an opaliser in glass manufacturing. Its unique physical property of having a refractive index close to water makes it nearly invisible when immersed. The Ivittuut deposit in Greenland was the world's only commercial source, and its protection during World War II underscored its strategic importance.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Cryolite (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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