Cryolite

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Cryolite (Na₃AlF₆, sodium aluminium fluoride) is an uncommon mineral of very limited natural distribution. It is mostly identified with the once large deposit at Ivigtût on the west coast of Greenland, which ran out in 1987.

It was historically used as an ore of aluminium and later in the electrolytic processing of the aluminium rich oxide ore, bauxite, which is a combination of aluminium oxide minerals such as gibbsite, boehmite and diaspore. The difficulty of removing aluminium from oxygen in the oxide ores was overcome by the use of cryolite as a flux in order to extract the aluminium metal. Now, as natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from fluorite for this purpose.

Cryolite occurs as glassy, colorless, white, reddish to grey-black prismatic monoclinic crystals. It has a Mohs hardness of 2.5 to 3 and a specific gravity of 2.95 to 3. It is translucent to transparent with very low refractive indices of a=1.3385-1.339, b=1.3389-1.339, g=1.3396-1.34. These RI values are very close to that of water and thus if immersed in water, cryolite becomes essentially invisible.

In addition to the Greenland occurrence, cryolite has been reported from Pikes Peak in Colorado, U.S.A.; Mont Saint-Hilaire, Montreal, Quebec, Canada; and at Miass, Russia. It is also known from Brazil, Czech Republic, Namibia, Norway, Ukraine, and several U.S. states.

Cryolite was first described in 1799 for an occurrence in Ivigtut and Arksukfiord, West Greenland. The name is derived from the Greek kryos = frost and lithos = stone.