| Cryolite | |
|---|---|
 Cryolite from Ivittuut, Greenland | |
| General | |
| Category | Halide mineral |
| Formula | Na3AlF6 |
| IMA symbol | Crl[1] |
| Strunz classification | 3.CB.15 |
| Dana classification | 11.6.1.1 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | P21/n |
| 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 |
| 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.[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. |
| References | [3][4][5][6][7] |
Cryolite (Na3AlF6,sodium hexafluoroaluminate) is a raremineral identified with the once-large deposit atIvittuut on the southwest coast ofGreenland, mined commercially until 1987.[8]
It is used in the reduction ("smelting") ofaluminium, in pest control, and as a dye.
Cryolite was first described in 1798 by Danish veterinarian and physicianPeter Christian Abildgaard [da] (1740–1801),[9][10] from rock samples obtained from localInuit who used the mineral for washing their hides; the actual source of the ore was later discovered in 1806 by the explorerKarl Ludwig Giesecke.[11][12] who found the deposit at Ivigtut (old spelling) and nearby Arsuk Fjord, Southwest Greenland, where it was extracted byØresund Chemical Industries.[13] The name is derived from theGreek wordscryos (Greek:κρύος,lit. 'frost'), andlithos (Greek:λίθος,lit. 'stone').[14]
ThePennsylvania Salt Manufacturing Company used large amounts of cryolite to makecaustic soda andfluorine compounds, includinghydrofluoric acid at itsNatrona, Pennsylvania, works, and at its integrated chemical plant inCornwells Heights, Pennsylvania, during the 19th and 20th centuries.[citation needed]
It was historically used as an ore of aluminium and later in the electrolytic processing of thealuminium-rich oxideorebauxite (itself a combination ofaluminium oxide minerals such asgibbsite,boehmite anddiaspore). The difficulty of separating aluminium from oxygen in the oxide ores was overcome by the use of cryolite as aflux to dissolve the oxide mineral(s).
Pure cryolite itself melts at 1012 °C (1285K), and it can dissolve the aluminium oxides sufficiently well to allow easy extraction of the aluminium byelectrolysis. 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 mineralfluorite.[citation needed]
In 1940 before enteringWorld 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.[15]
In 1987 the main mining in Ivittuut was closed. According toeconomist Arindam Banerjee, exploitation of cryolite in Greenland contributed nearly 54 billion euros to Danish economy,[16] though this claim has been strongly disputed.[17]
BesidesIvittuut, on the southwest coast ofGreenland where cryolite was once found in commercial quantities, small deposits of cryolite have also been reported in some areas ofSpain, at the foot ofPikes Peak inColorado, Francon Quarry nearMontreal inQuebec,Canada and also in Miask,Russia.[18][19]
Molten cryolite is used as a solvent foraluminium oxide (Al2O3) in theHall–Héroult process, used in the refining ofaluminium. It decreases the melting point of aluminium oxide from 2000–2500 °C to 900–1000 °C, and increases its conductivity[20] thus making the extraction of aluminium more economical.[21]
Cryolite is used as aninsecticide and apesticide.[22] It is also used to givefireworks a yellow color.[23]It is used in glass manufacturing as a "powerful opaliser."[24]
Cryolite occurs as glassy, colorless, white-reddish to gray-black prismaticmonoclinic crystals. It has aMohs hardness of 2.5 to 3 and aspecific gravity of about 2.95 to 3.0. It is translucent to transparent with a very lowrefractive index of about 1.34, which is very close to that ofwater; thus if immersed in water, cryolite becomes essentially invisible.[7]
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.)
[From p. 312] Han har kaldt denne grönlandske SteenKryolith ellerIissteen formedelst dens Udseende, og fordi den smelter saa meget let for Blæsröret.(He has named this Greenlandic stonecryolite orice stone on account of its appearance, and because it melts so easily under a blowpipe.)
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