Cryolite

noun

1. An uncommon, white, vitreous natural fluoride of aluminum and sodium, Na3AlF6, nearly invisible in water in powdered form and used chiefly in the electrolytic recovery of aluminum.
2. A fluorid of sodium and aluminium found in Greenland, where it forms an extensive bed.
3. A fluoride of sodium and aluminum, found in Greenland, in white cleavable masses; -- used as a source of soda and alumina.
4. The mineral sodium aluminium fluoride (Na3AlF6).
5. a white mineral consisting of fluorides of aluminum and sodium; a source of fluorine