A mineral with chemical composition Na₃AlF₆. Although it crystallizes in the monoclinic system, cryolite has three directions of parting at nearly 90°, giving it a pseudocubic aspect. Hardness is 2½ on Mohs scale and the specific gravity is 2.95. Crystals are usually snow-white but may be colorless and more rarely brownish, reddish, or even black (see illus.). The mean refraction index is 1.338, approximately that of water, and thus fragments become invisible when immersed in water. Cryolite, associated with siderite, galena, and chalcopyrite, was discovered at Ivigtut, Greenland, in 1794. This locality remains the only important occurrence.