A colourless monoclinic solid, Na3AlF6, very slightly soluble in water; r.d. 2.9; m.p. 1000°C. It changes to a cubic form at 580°C. The compound occurs naturally as the mineral cryolite but a considerable amount is manufactured by the reaction of aluminium fluoride wth alumina and sodium hydroxide or directly with sodium aluminate. Its most important use is in the manufacture of aluminium in the Hall-Heroult cell. It is also used in the manufacture of enamels, opaque glasses, and ceramic glazes.