Cryolite

Cryolite (Na3AlF6, sodium hexafluoroaluminate) is an uncommon mineral 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 (itself a combination of aluminium oxide minerals such as gibbsite, boehmite and diaspore). 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). Cryolite itself melts below 900°C (1173 Kelvin) and can dissolve aluminium oxides sufficiently well to allow easy extraction of the aluminium by electrolysis. Considerable energy is still required for both heating the materials and the electrolysis, but it is much more energy-efficient than melting the oxides themselves. Now, as natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral 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.

Cryolite has also been reported at Pikes Peak, Colorado; Mont Saint-Hilaire, Quebec; and at Miass, Russia. It is also known in 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 cryò = chill and lithòs = stone.