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Cryolite (Na3AlF6, sodium aluminium fluoride.) is an uncommon mineral of very limited natural distribution. Mostly considered a one locality mineral, for although there are a few other minor localities, it was only found in large quantities on the west coast of Greenland.
It was used as a solvent of the aluminium rich ore, bauxite, which is a combination of aluminium oxides such as gibbsite, boehmite and diaspore. It is very difficult to remove atoms of aluminium from atoms of oxygen which is necessary in order to produce aluminium metal. Cryolite made an excellent flux to make the process less expensive. Now it is too rare to be used for this purpose and sodium aluminium fluoride is produced artificially to fill the void.
A curious note about cryolite is the fact that it has a low index of refraction close to that of water. This means that if immersed in water, a perfectly clear colorless crystal of cryolite or powdered cryolite will essentially disappear. Even a specimen of cloudy cryolite will become more transparent and its edges will be less distinct, an effect similar to ice in water except that the ice floats.
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