Cryolite

Cryolite is a rare mineral made of sodium, aluminium and fluorine (Na₃AlF₆), white or colourless and ice-like in look. Its name comes from the Greek for “ice” and “stone”. For a long time it came almost only from the Ivittuut mine in Greenland and served in making aluminium by electrolysis, as the molten bath that dissolves aluminium oxide. Because the natural deposit is exhausted, industry today uses synthetic cryolite.

Aluminium production: In the Hall–Héroult process, aluminium oxide is dissolved in a molten cryolite bath. This brings the melting temperature down from over 2,000 °C to about 900–1,000 °C; according to Wikipedia it also improves the bath’s conductivity. Today synthetic, not natural, cryolite is used for this. Other uses: As an insecticide, as an additive that turns glass milky white (an opacifier), to give fireworks a yellow colour, and in the 19th and 20th centuries in Pennsylvania to make caustic soda and fluorine compounds such as hydrofluoric acid.

It has a hardness of 2.5–3 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 2.95–3 g/cm³.

Its best-known localities: Ivittuut (Ivigtut), Sermersooq, southwest Greenland; Miass, Ilmen Mountains, Southern Urals, Russia; Khibiny Mountains, Kola Peninsula, Russia; Mont Saint-Hilaire, Quebec, Canada; Pikes Peak area (St. Peters Dome), Colorado, USA.

It is white or colourless, with a glassy, sometimes greasy or waxy lustre. Crystals are rare; it usually occurs as coarse-grained masses. It is very soft (hardness 2.5–3) and its specific gravity is about 3. Its refractive index is very close to that of water (about 1.34), so clear fragments almost vanish when dropped into water. In Ivittuut specimens it is usually intergrown with siderite, galena, chalcopyrite or fluorite. Because natural cryolite has run out, the cryolite used in industry is synthetic, made from raw materials such as fluorite. Anyone looking for a natural specimen should ask where the stone came from (for example Ivittuut).

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. In industry natural cryolite is no longer mined; the value of the synthetic cryolite used by the aluminium industry depends on its purity and the cost of its raw materials. Among collectors the most sought-after specimens come from Ivittuut, whose mine closed in 1987. Crystal form (rare), size, freedom from damage and an attractive association with minerals such as siderite, galena or chalcopyrite raise the value. Cryolite is too soft to be used as a gemstone.

Crystal-therapy websites claim that cryolite helps with speech difficulties and regulates bodily functions, and that it stimulates the brain and makes learning easier. The same sites say cryolite opens the “third-eye chakra”, strengthens intuition, and brings happiness and peace. No study supports these claims. Cryolite is a fluorine compound; breathing its dust for a long time damages the bones (seen in cryolite workers). It should therefore never be ground up and taken, or soaked in drinking water. Someone with a speech problem or learning difficulty should consult a specialist. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.

Cryolite contains fluorine and aluminium. Handling a specimen is no problem, but if cryolite dust is breathed for years it can cause skeletal fluorosis, in which the bones become denser and harder and pain and stiffness set in; the disease was first recognised in workers who crushed and refined cryolite. Never grind or sand a specimen, do not breathe its dust, never put it in your mouth, and never put it in drinking water.