Cryolite (
Na3AlF6,
sodium hexafluoroaluminate) is an uncommon
mineral identified with the once large deposit at
Ivigtût on the west coast of
Greenland, depleted by 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). Pure cryolite itself melts at 1012 °C (1285
K), and it can dissolve the aluminium oxides sufficiently well to allow easy extraction of the aluminium by
electrolysis. Substantial energy is still needed for both heating the materials and the electrolysis, but it is much more energy-efficient than melting the oxides themselves. As natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral
fluorite. Cryolite occurs as glassy, colorless, white-reddish to gray-black prismatic
monoclinic crystals. It has a
Mohs hardness of 2.5 to 3 and a
specific gravity of about 2.95 to 3.0. It is translucent to transparent with a very low
refractive index of about 1.34, which is very close to that of
water; thus if immersed in water, cryolite becomes essentially invisible.