Description
Cryolite (Na3AlF6, sodium hexafluoroaluminate, cryolite) is a relatively rare mineral known from only a few locations worldwide. The best-known historical deposit is the once-large Ivigtût mine on the west coast of Greenland. This mine closed in 1987. Historically, cryolite was used as an ore in aluminum production and later in the electrolytic processing of aluminum-rich oxides, such as the well-known bauxite, which is a combination of several minerals such as gibbsite, boehmite, and diaspore, all rich in aluminum oxide. The difficult decomposition of aluminum oxide into metal and oxygen was solved by using cryolite as a flux additive. Now that natural sources of cryolite are depleted, synthetic sodium aluminum fluoride from fluorite is used. Cryolite occurs as a glassy mineral with hues ranging from colorless or white to reddish or grayish-black. The crystalline form is prismatic monoclinic. Its hardness is 2.5 to 3 on the Mohs scale and its density is 2.95 to 3.00. Cryolite is translucent to transparent with a particularly low refractive index. Its refractive index values range from 1.3385 to 1.34, very close to that of water. Therefore, when immersed in water, cryolite is practically invisible. Besides the Greenland deposit, cryolite is also found in Pikes Peak (USA), Mont Saint-Hilaire (Canada), and Miass (Russia). Smaller deposits occur in Brazil, the Czech Republic, Namibia, Norway, Ukraine, and several other states in the United States. Cryolite was first described in 1799 based on its occurrence in Ivigtût and Arksukfiord in West Greenland. The name is derived from the Greek kryos = frozen and lithos = stone.
