cryolite

{{Infobox mineral | name = Cryolite | category = Halide mineral | boxwidth = 24 | boxbgcolor = | image = 816- Ivigtut - cryolite.jpg | imagesize = 260px | caption = Cryolite from Ivittuut, Greenland | formula = Na3AlF6 | molweight = 209.9 g mol−1 | IMAsymbol=Crl | strunz = 3.CB.15 | dana = 11.6.1.1 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/n | unit cell = a = 7.7564(3) Å, b = 5.5959(2) Å, c = 5.4024(2) Å; β = 90.18°; Z = 2 | color = Colorless to white, also brownish, reddish and rarely black | habit = Usually massive, coarsely granular. The rare crystals are equant and pseudocubic. | lattice = | twinning = Very common, often repeated or polysynthetic with simultaneous occurrence of several twin laws | cleavage = None observed | fracture = Uneven | tenacity = Brittle | mohs = 2.5 to 3 | luster = Vitreous to greasy, pearly on {001} | refractive = nα = 1.3385–1.339, nβ = 1.3389–1.339, nγ = 1.3396–1.34 | opticalprop = Biaxial (+) | birefringence = δ = 0.001 | dispersion = r 3 solution, soluble in H2SO4 with the evolution of HF, which is poisonous. Insoluble in water. | diaphaneity = Transparent to translucent | other = 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 a rare mineral identified with the once-large deposit at Ivittuut on the southwest coast of Greenland, mined commercially until 1987.

It is used in the reduction ("smelting") of aluminium, in pest control, and as a dye.