Cryolite

Cryolite is a sodium aluminium fluoride (Na3AlF6), historically important as the flux that made aluminium smelting possible. Its name, from the Greek for 'frost stone', reflects its icy white appearance and an unusually low refractive index that lets crushed crystals nearly disappear when placed in water.

Properties

Cryolite is monoclinic but forms pseudo-cubic crystals and granular masses; it is soft (2.5–3) and colourless to white, sometimes tinted by inclusions. Most museum specimens come from a single legendary deposit.

Occurrence

The great commercial source was Ivigtut (Ivittuut) in south-west Greenland, a massive cryolite body that was mined out in the twentieth century, after which the smelting industry switched to synthetic cryolite. Minor occurrences are known at Miask in the Urals and at Pikes Peak, Colorado. Well-crystallised natural cryolite is now scarce and collectable.