Cryolite ( Na₃ Al F₆, sodium hexafluoroaluminate) is a rare

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

identified with the once-large deposit at Ivittuut on the west coast of

Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...

, mined commercially until 1987. It is used in the reduction ("

smelting Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...

") of aluminium, in pest control, and as a dye.

History

Cryolite was first described in 1798 by Danish veterinarian and physician (1740–1801), from rock samples obtained from local

Inuit Inuit (singular: Inuk) are a group of culturally and historically similar Indigenous peoples traditionally inhabiting the Arctic and Subarctic regions of North America and Russia, including Greenland, Labrador, Quebec, Nunavut, the Northwe ...

who used the mineral for washing their hides; the actual source of the ore was later discovered in 1806 by the explorer Karl Ludwig Giesecke. who found the deposit at Ivigtut (old spelling) and nearby Arsuk Fjord, Southwest Greenland, where it was extracted by Øresund Chemical Industries. The name is derived from the Greek words (), and (). The Pennsylvania Salt Manufacturing Company used large amounts of cryolite to make caustic soda and

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

compounds, including hydrofluoric acid at its Natrona, Pennsylvania, works, and at its integrated chemical plant in Cornwells Heights, Pennsylvania, during the 19th and 20th centuries. It was historically used as an ore of aluminium and later in the electrolytic processing of the

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

-rich oxide ore

bauxite Bauxite () is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH) ...

(itself a combination of

aluminium oxide Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several Aluminium oxide (compounds), aluminium oxides, and specifically identified as alum ...

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 In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

. 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. 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. In 1987 the main mining in Ivittuut was closed. According to economist Arindam Banerjee, exploitation of cryolite in Greenland contributed to nearly 54 billion euros to Danish economy, though this claim has been strongly disputed.

Source locations

Besides Ivittuut, on the west coast of

where cryolite was once found in commercial quantities, small deposits of cryolite have also been reported in some areas of

Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...

, at the foot of

Pikes Peak Pikes Peak is the List of mountain ranges of Colorado#Mountain ranges, highest summit of the southern Front Range of the Rocky Mountains in North America. The Ultra-prominent peak, ultra-prominent fourteener is located in Pike National Forest, ...

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

, Francon Quarry near

Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...

Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

and also in Miask,

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

Uses

Molten cryolite is used as a solvent for

(Al₂O₃) in the

Hall–Héroult process The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) (obtained most often from bauxite, aluminium's chief ore, through the Bayer process) in molten cryolite and e ...

, used in the refining of

. It decreases the melting point of aluminium oxide from 2000–2500 °C to 900–1000 °C, and increases its conductivity thus making the extraction of aluminium more economical. Cryolite is used as an

insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...

and a

pesticide Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...

. It is also used to give fireworks a yellow color. It is used in glass manufacturing as a "powerful opaliser."

Physical properties

Cryolite occurs as glassy, colorless, white-reddish to gray-black prismatic

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

crystals. It has a

Mohs hardness The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...

of 2.5 to 3 and a

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

of about 2.95 to 3.0. It is translucent to transparent with a very low

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

of about 1.34, which is very close to that of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

; thus if immersed in water, cryolite becomes essentially invisible.

References

''Phase transitions and volumetric properties of cryolite, Na₃AlF₆: Differential thermal analysis to 100 MPa;'' American Mineralogist; January 2006; v. 91; no. 1; p. 97-103
{{Authority control Aluminium minerals History of Greenland Natural history of Greenland Pyrotechnic colorants Sodium minerals Monoclinic minerals Minerals in space group 14 Fluorine minerals Inorganic insecticides Pesticides