Cryolite

Chemical formula: Na<sub>3</sub>AlF<sub>6</sub>

Cryolite is a rare sodium aluminum fluoride, historically important as an aluminum ore, known for its ice-like appearance and low refractive index.

## Characteristics Cryolite is a sodium aluminum fluoride that forms prismatic crystals, often twinned to create pseudo-cubic forms. It typically occurs as coarse-grained, coherent masses that can reach considerable sizes. Its name comes from the Greek words for "ice" and "stone," which is an apt reference to its appearance. Pure cryolite is colorless and transparent, and when immersed in water, it becomes almost invisible due to its refractive index (approx. 1.338) being very close to that of water. ## Physical Properties This mineral is relatively soft, with a hardness of 2.5 on the Mohs scale, meaning it can be scratched with a fingernail. It has a vitreous luster, and on cleavage surfaces, it can be pearly or greasy. It is brittle, and its density is approximately 2.95-3.0 g/cm³. It exhibits weak thermoluminescence and fluorescence under ultraviolet light. ## Colors and Varieties Cryolite is most commonly colorless, white, or grayish. It can also take on yellowish, reddish-brown, or even black hues due to inclusions of other minerals such as galena, chalcopyrite, or siderite. No named commercial or gemological varieties are distinguished. ## History and Name The name "cryolite" was given in 1799 by the Danish physician and veterinarian Peder Christian Abildgaard. It comes from the Greek words κρύος (kryos) - ice and λίθος (lithos) - stone, referring to its icy appearance. This mineral was first discovered in Ivittuut (formerly Ivigtut) in Greenland. ## Uses Historically, cryolite was the only aluminum ore of industrial significance. Its primary use was as a flux in the Hall-Héroult process for the electrolytic production of aluminum from aluminum oxide (bauxite). It lowered the melting temperature of aluminum oxide, making the process more economical. Currently, due to the depletion of the only commercial deposit, synthetic sodium aluminum fluoride is used in industry. Cryolite also has limited use in the production of some abrasives, enamels, and milky glass.

Properties

Mohs hardness
2.5
Luster
Vitreous, Pearly, Greasy
Streak
White
Density
2.95-3.0
Cleavage
Poor on {001} and {110}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification A key diagnostic feature of cryolite is its low refractive index, very close to that of water. A fragment of the mineral placed in water becomes almost invisible. Other features include low hardness (2.5 on the Mohs scale), vitreous or greasy luster, and characteristic twinning resembling cubes. It melts easily in a candle flame. ## Distinguishing from Similar Minerals Cryolite is sometimes confused with calcite, quartz, or fluorite. It is distinguished from quartz by its significantly lower hardness. It is distinguished from calcite by its lack of reaction with hydrochloric acid. From fluorite, which is also a fluoride, it is distinguished by its crystal form, poorer cleavage, and the characteristic "disappearing" in water. ## Crystal Forms Cryolite crystallizes in the monoclinic system, but its crystals are strongly pseudo-regular and often form complex twins that mimic regular system forms, such as cubes and octahedra. It most commonly occurs as coherent, coarse-grained masses.

Geological environment

## Genesis Cryolite is a mineral of magmatic origin, associated with the late stages of crystallization of alkaline granites and pegmatites, rich in sodium and fluorine. It forms under hydrothermal conditions. The world's only industrially significant deposit, in Ivittuut, Greenland, was in the form of a large pegmatite mass within a granitic intrusion. ## Mineral Associations This mineral often co-occurs with other rare fluorides. Typical associated minerals include siderite, quartz, fluorite, galena, chalcopyrite, as well as rarer ones like pachnolite, thomsenolite, chiolite, and weberite. ## Localities Historically, the most important and only industrially exploited locality was the pegmatite complex in Ivittuut (Ivigtut) on the west coast of Greenland. This deposit is now completely exhausted. Small, non-commercial occurrences of cryolite have also been reported in Pikes Peak, Colorado (USA), in the Mont Saint-Hilaire massif in Quebec (Canada), and in the Urals, Russia.

Rarity

Rare

For collectors

## Quality Criteria Well-formed, sharp crystals are most valued by collectors, which is a great rarity. Transparent, colorless specimens with a strong luster are sought after. Associations with other minerals, especially color-contrasting siderite, enhance their attractiveness. Large, pure masses with an icy appearance are also desirable. ## Popular Localities The vast majority, if not all, of the best cryolite specimens come from the historic deposit in Ivittuut, Greenland. Material from this locality is considered classic and most valuable. Specimens from other places, such as Mont Saint-Hilaire or Pikes Peak, are extremely rare and valued mainly by collectors specializing in minerals from those localities.

Care and storage

## Cleaning Cryolite is a very delicate and soft mineral. For cleaning, use only distilled water and a very soft brush. Avoid ultrasonic cleaners, which can cause fractures. ## What to Avoid The mineral is sensitive to rapid temperature changes, which can lead to fractures. Avoid contact with acids and strong chemicals. Due to its low hardness, it is very susceptible to scratches and mechanical damage. ## Storage Cryolite specimens should be stored separately in padded boxes to prevent scratching by harder minerals. It should not be exposed to dust or moisture. It is best displayed in enclosed cabinets.

External references

Sources

Read more