cryolite

A mineral with chemical composition Na₃AlF₆. It crystallizes in the monoclinic system. Hardness is 2½ on Mohs scale and the specific gravity is 2.95. Crystals are usually snow-white but may be colorless and more rarely brownish, reddish, or even black. The mean refraction index is 1.338, approximately that of water, and thus fragments become invisible when immersed in water.

Cryolite was once used as a source of metallic sodium and aluminum, but now is used chiefly as a flux in the electrolytic process in the production of aluminum from bauxite. See also Aluminum.

A fluoride often used as a flux in the manufacture of silicate cements.

Environment

Extremely rare in pegmatite dikes.

The monoclinic crystals are usually in subparallel growths on a solid cryolite surface and look like cubes, sometimes with pseudo-octahedral truncations. Also massive.

White or colorless. Luster glassy or greasy; hardness 2Ɖ; specific gravity 2.9-3.0; fracture uneven; cleavage none, but pseudocubic partings. Brittle; translucent.

Fluoride of sodium and aluminum (32.8% Na, 12.8% Al, 54.4% F).

Fuses very easily on charcoal with a yellow (sodium) coloration of the flame. Bead that forms is clear when hot, white when cold, and fluoresces blue-green in shortwave ultraviolet light.

This is practically a one-locality mineral. White massive specimens, with brown siderite, from the one important locality should be easily recognized. Failing that, the fusion test is sufficient.

A strange mineral, surprisingly uncommon in nature. The only important locality was a unique, now worked-out pegmatite in Greenland, where the cryolite formed great solid masses, sometimes with fissures lined with crystals of cryolite or of some other related mineral. Embedded in it, and common in cryolite specimens, are chalcopyrite, siderite, and galena. It was mined for use as the solvent of bauxite aluminum ore, for the electrolytic recovery of aluminum. The insignificant U.S. occurrence is in Creede, Colorado. Cryolite is made artificially from fluorite for aluminum electrolysis.

This mineral has a very slight ability to bend light (refraction); it is close to water in that respect. Consequently, cryolite powder put in water comes so close to the liquid in its refraction of light that the powder becomes almost invisible.

A naturally occurring mineral, sodium aluminum fluoride, May be a source of fluorine poisoning if the mineral is used industrially.

Cryolite from Greenland

The cryolite mine Ivigtut, Greenland, summer 1940

Cryolite's unit cell

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). Pure cryolite itself melts at 1012°C (1285 Kelvin), and it can dissolve the 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.

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 about 2.95 to 3.0. 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 it is 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 small quantities in Brazil, the Czech Republic, Namibia, Norway, Ukraine, and several American states.

Cryolite was first described in 1799 from a deposit of it in Ivigtut and Arksukfiord, West Greenland. The name is derived from the Greek language words cryò = chill, and lithòs = stone.

Cryolite has also been used as a pesticide.^[1]

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
• Webmineral data
• Mindat with location data
• Mineral galleries
• Greek-English-Greek dictionary
• Phase transitions and volumetric properties of cryolite, Na3AlF6: Differential thermal analysis to 100 MPa

Footnotes

1. ^ Lawrenz, M.; Mitchell, H. H.; Ruth, W. A. (1939). "The Comparative Toxicity of Fluorine in Calcium Fluoride and in Cryolite". Journal of Nutrition (Am Soc Nutrition) 18 (2): 115. http://jn.nutrition.org/cgi/reprint/18/2/115. 

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