Cryolite

Cryolite

Cryolite from Ivigtut Greenland
General
Category Halide Mineral
Formula
(repeating unit)		 Na3AlF6
Strunz classification 03.CB.15
Dana classification 11.6.1.1
Crystal symmetry Monoclinic 2/m
Unit cell a = 7.7564(3) Å, b = 5.5959(2) Å, c = 5.4024(2) Å; β = 90.18°; Z = 2
Identification
Formula mass 209.9 g mol−1
Color Colorless to white, also brownish, reddish and rarely black
Crystal habit Usually massive, coarsely granular. The rare crystals are equant and pseudocubic
Crystal system Monoclinic 2/m
Twinning Very common, often repeated or polysynthetic with simultaneous occurrence of several twin laws
Cleavage None observed
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 2.5 to 3
Luster Vitreous to greasy, pearly on {001}
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.95 to 3.0.
Optical properties Biaxial (+)
Refractive index nα = 1.3385–1.339, nβ = 1.3389–1.339, nγ = 1.3396–1.34
Birefringence δ = 0.001
2V angle 43°
Dispersion r < v
Melting point 1012 °C
Solubility Soluble in AlCl3 solution, soluble in H2SO4 with the evolution of HF, which is poisonous. Insoluble in water.[1]
Other characteristics Weakly thermoluminescent. Small clear fragments become nearly invisible when placed in water, since its refractive index is close to that of water. May fluoresce intense yellow under SWUV, with yellow phosphorescence, and pale yellow phosphorescence under LWUV. Not radioactive.
References [2][3][4][5][6]

Cryolite (Na3AlF6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987.

The cryolite mine Ivigtut, Greenland, summer 1940

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. Now, as natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral fluorite.

Cryolite's unit cell

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.[6]

Cryolite has also been reported at Pikes Peak, Colorado; Mont Saint-Hilaire, Quebec; and at Miass, Russia. It is also known in small quantities in Brazil, the Czech Republic, Namibia, Norway, Ukraine, and several American states.

Cryolite was first described in 1799 from a deposit of it in Ivigtut and nearby Arsuk Fjord, Southwest Greenland. The name is derived from the Greek language words cryò = chill, and lithòs = stone.[3] The Pennsylvania Salt Manufacturing Company used large amounts of cryolite to make caustic soda at its Natrona, Pennsylvania works during the 19th and 20th centuries.

Uses

Cryolite is used as an insecticide and a pesticide.[7] It is also used to give fireworks a yellow color.[8] Molten cryolite is used as a solvent for aluminium oxide (Al2O3) in the Hall–Héroult process, used in the refining of aluminum.

References

  1. ↑ CRC Handbook of Chemistry and Physics, 83rd Ed., p. 4-84.
  2. ↑ Gaines, Richard V., et al (1997) Dana’s New Mineralogy, Wiley, 8th, ISBN 978-0-471-19310-4
  3. ↑ 3.0 3.1 Cryolite: Cryolite mineral information and data. Mindat.org (2010-10-03). Retrieved on 2010-10-25.
  4. ↑ Cryolite Mineral Data. Webmineral.com. Retrieved on 2010-10-25.
  5. ↑ Cryolite, Handbook of Mineralogy. Retrieved on 2010-10-25.
  6. ↑ 6.0 6.1 Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
  7. ↑ EPA R.E.D. FACTS Cryolite http://www.epa.gov/oppsrrd1/REDs/factsheets/0087fact.pdf
  8. ↑ http://chemistry.about.com/od/fireworkspyrotechnics/a/fireworkcolors.htm