Cryolite

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.

Molten cryolite is used as a solvent for aluminium oxide (Al2O3) in the Hall–Héroult process, used in the refining of aluminium. It decreases the melting point of molten (liquid state) aluminium oxide from 2000 to 2500 °C to 900–1000 °C, and increases its conductivity thus making the extraction of aluminium more economical. Cryolite is used as an insecticide and a pesticide. It is also used to give fireworks a yellow color.

In traditional folklore, cryolite was considered a stone of protection. During ancient times, people believed that cryolite warded off evil spirits, so they would wear it around their chests, often set in gold, to deter phantoms and thus protect themselves.

Cryolite was first described in 1798 by Danish veterinarian and physician Peder Christian Abildgaard (1740–1801); it was obtained from a deposit of it in Ivigtut and nearby Arsuk Fjord, Southwest Greenland. The name is derived from the Greek language words κρύος (cryos) = frost, and λίθος (lithos) = stone. The Pennsylvania Salt Manufacturing Company used large amounts of cryolite to make caustic soda at its Natrona, Pennsylvania works, and at its Cornwells Heights, Pennsylvania, Plant, 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 (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.