<Tr> <Th> References </Th> <Td> </Td> </Tr> <P> Cryolite (Na Al F, sodium hexafluoroaluminate) is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987 . </P> <P> 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 . </P> <P> 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 . </P>

Why is molten cryolite used in the extraction of aluminium
find me the text answering this question