THE MINERAL CRYOLITE

Chemistry: Na3AlF6, Sodium Aluminum Fluoride
Class: Halides
Uses: as a aid to aluminum processing and other industrial uses and as mineral specimens.
Specimens

Cryolite is an uncommon mineral of very limited natural distribution. Mostly considered a one locallity mineral, for although there are a few other minor locallities, it was only found in large quantities on the west coast of Greenland.

It was used as a solvent of the aluminum rich ore, bauxite, which is a combination of aluminum oxides such as gibbsite, boehmite and diaspore. It is very difficult to remove atoms of aluminum from atoms of oxygen which is necessary in order to produce aluminum 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 aluminum 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 emersed 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.

PHYSICAL CHARACTERISTICS:

Color is clear or white to yellowish, but can also be black or purple.
Luster is vitreous.
Transparency crystals are transparent to translucent.
Crystal System is monoclinic; 2/m
Crystal Habits are usually massive and as pseudo-cubic crystals, some with psuedo-octahedral truncations.
Cleavage is absent, but three parting directions produce what looks like a psuedo-cubic cleavage.
Fracture is uneven.
Hardness is 2.5 - 3
Specific Gravity is 2.95 (average)
Streak is white.
Other Characteristics: index of refraction is 1.338 which is close to the index of refraction of water. As a consequence, clear cryolite crystals or powdered cryolite will nearly disappear in water. Also there is no salty taste which is helpful in distinguishing cryolite from the mineral halite.
Associated Minerals include siderite, quartz, topaz, fluorite, chalcopyrite, galena, cassiterite, molybdenite, columbite and wolframite.
Notable Occurances include Ivigtut area of Greenland and also at the foot of Pikes Peak at Creede, Colorado, USA, Mont Saint-Hilaire and Francon Quarry, Montreal, Quebec, Canada and at Miask, Russia.
Best Field Indicators are lack of salty taste, density, index of refraction, locallity and crystal habit.