Soluble in AlCl3 solution, soluble in H2SO4 with the evolution of HF, which is poisonous. Insoluble in water.[2]
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.
It was historically used as an ore of aluminium and later in the electrolytic processing of the aluminium-rich oxide orebauxite (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 now too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral fluorite.[citation needed]
In 1940 before entering World War II, the United States became involved with protecting the world's largest cryolite mine in Ivittuut, Greenland from falling into Nazi Germany's control.[13]
Besides Ivittuut, on the west coast of Greenland where cryolite was once found in commercial quantities, small deposits of cryolite have also been reported in some areas of Spain, at the foot of Pikes Peak in Colorado, Francon Quarry near Montreal in Quebec, Canada and also in Miask, Russia.[14][15]
Uses
Molten cryolite is used as a solvent for aluminium oxide (Al2O3) in the Hall–Héroult process, used in the refining of aluminium. It decreases the melting point of aluminium oxide from 2000–2500 °C to 900–1000 °C, and increases its conductivity[16] thus making the extraction of aluminium more economical.[17]
Cryolite's unit cell. Fluorine atoms (yellow) are arranged in octahedra around aluminium atoms (red). Sodium ions (purple) occupy the interstices between the octahedra.
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.[7]
^CRC Handbook of Chemistry and Physics, 83rd Ed., p. 4–84.
^Gaines, Richard V.; Skinner, H. Catherine W.; Foord, Eugene E.; Mason, Brian; Rosensweig, Abraham (1997). Dana's new mineralogy : the system of mineralogy of James Dwight Dana and Edward Salisbury Dana (8th, entirely rewritten and greatly enl. ed.). New York: Wiley. ISBN978-0-471-19310-4.
^Abildgaard (1799). "Norwegische Titanerze und andre neue Fossilien" [Norwegian titanium ores and other new fossils [i.e., anything dug out of the earth])]. Allgemeines Journal der Chemie (in German). 2: 502. In der ordentlichen Versammlung der königl. Gesellschaft der Wissenschaften am 1. Februar dieses Jahres stattete Hr. Prof. Abildgaard einen Bericht über die Norwegischen Titanerze und über die von ihm mit denselben angestellten Analysen ab. Zugleich theilte er auch eine Nachricht von einer vor wenigen Jahren aus Grönland nach Dänemark gebrachten besonders weißen spathartigen Miner mit. Einer damit angestellten Untersuchung zu folge bestand sie aus Thonerde und Flußspathsäure. Eine Verbindung, von welcher noch kein ähnliches Beyspiel im Mineralreich vorgekommen ist. Sie hat den Namen Chryolit erhalten, weil sie vor dem Löthrohre wie gefrorne Salzlauge schmilzt. (At the ordinary session of the [Danish] Royal Society of Science on February 1st of this year, Prof. Abildgaard presented a report about Norwegian titanium ores and about the analysis of them undertaken by him. He also communicated a notice of an especially white, spar-like mineral that was brought several years ago from Greenland to Denmark. According to an investigation performed on it, it consists of alumina and hydrofluoric acid. A compound of which no similar example in the mineral realm has yet been found. It received the name "cryolite" because under a blowpipe, it melts like frozen brine.)
^Abildgaard, P. C. (1800). "Om Norske Titanertser og om en nye Steenart fra Grönland, som bestaaer af Flusspatsyre og Alunjord" [On Norwegian titanium ores and on a new mineral from Greenland, which consists of hydrofluoric acid and alumina]. Det Kongelige Danske Videnskabers-Selskabs (The Royal Danish Scientific Society). 3rd series (in Danish). 1: 305–316. [From p. 312] Han har kaldt denne grönlandske Steen Kryolith eller Iissteen formedelst dens Udseende, og fordi den smelter saa meget let for Blæsröret. (He has named this Greenlandic stone cryolite or ice stone on account of its appearance, and because it melts so easily under a blowpipe.)
d'Andrada (1800). "Kurze Angabe der Eigenschaften und Kennzeichen einiger neuen Fossilien aus Schweden und Norwegen, nebst einigen chemischen Bemerkungen über dieselben" [Brief statement of the properties and characteristics of some new minerals from Sweden and Norway, together with some chemical observations about the same]. Allgemeine Journal der Chemie (in German). 4: 28–39. Cryolite (Chryolit) is discussed on pp. 37–38. From p. 38: "Dieses sonderbare Fossil besteht aus Thonerde, Fluẞspathsäure und ein klein wenig Kali. Er kommt vor in Grönland, … " (This strange mineral consists of alumina, hydrofluoric acid and a very little potassium carbonate. It occurs in Greenland, … )
^Albert Huntington Chester, A Dictionary of the Names of Minerals Including Their History and Etymology (New York, New York: John Wiley & Sons, 1896), p. 68.
^Totten, George E.; MacKenzie, D. Scott (2003). Handbook of Aluminum: Volume 2: Alloy production and materials manufacturing. vol. 2. New York, NY: Marcel Dekker, Inc. ISBN0-8247-0896-2.
