Cryolite

Cryolite
Cryolite from Ivigtut Greenland
General
Category	Halide Mineral
Formula (repeating unit)	Na3•AlF6
Strunz classification	3.CB.15
Dana classification	11.6.1.1
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 7.7564(3) Å, b = 5.5959(2) Å, c = 5.4024(2) Å; β = 90.18°; Z = 2
Identification
Formula mass	209.9 g mol−1
Color	Colorless to white, also brownish, reddish and rarely black
Crystal habit	Usually massive, coarsely granular. The rare crystals are equant and pseudocubic
Twinning	Very common, often repeated or polysynthetic with simultaneous occurrence of several twin laws
Cleavage	None observed
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	2.5 to 3
Luster	Vitreous to greasy, pearly on {001}
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.95 to 3.0.
Optical properties	Biaxial (+)
Refractive index	nα = 1.3385–1.339, nβ = 1.3389–1.339, nγ = 1.3396–1.34
Birefringence	δ = 0.001
2V angle	43°
Dispersion	r < v
Melting point	1012 °C
Solubility	Soluble in AlCl3 solution, soluble in H2SO4 with the evolution of HF, which is poisonous. Insoluble in water.
Other characteristics	Weakly thermoluminescent. Small clear fragments become nearly invisible when placed in water, since its refractive index is close to that of water. May fluoresce intense yellow under SWUV, with yellow phosphorescence, and pale yellow phosphorescence under LWUV. Not radioactive.
References

Cryolite (Na₃AlF₆, sodium hexafluoroaluminate) is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 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 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.

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.