Carnallite

Chemical formula: KMgCl<sub>3</sub>·6H<sub>2</sub>O

Carnallite is a hydrated potassium and magnesium chloride, found in marine evaporite deposits, valued primarily as a raw material for the production of potassium and magnesium.

## Characteristics Carnallite is a mineral from the halide group, chemically classified as a hydrated potassium and magnesium chloride. It rarely forms distinct pseudohexagonal or rhombic crystals, most often occurring as compact, granular, or fibrous aggregates. It is a very hygroscopic mineral, meaning it readily absorbs moisture from the air, leading to its gradual dissolution and loss of form. Pure carnallite is colorless or white, but iron oxide impurities can impart yellowish, reddish-brown, or even blue or violet hues. ## Physical Properties Carnallite is characterized by low hardness, 2.5 on the Mohs scale, meaning it can be scratched with a fingernail. It has a vitreous luster, and on fresh surfaces, a greasy luster. It is transparent to translucent. Its density is low, approximately 1.6 g/cm³, a value similar to that of some plastics. A characteristic feature is its bitter, salty taste and solubility in water. ## Colors and Varieties Carnallite is most commonly found colorless, white, yellowish, or reddish. The presence of hematite as fine inclusions causes a red coloration. Rarer specimens exhibit blue or violet coloration, whose genesis is similar to that in halite and sylvite – related to crystal lattice defects caused by natural radiation. ## History and Name The mineral was named in 1856 in honor of the Prussian mining engineer, Rudolf von Carnall (1804–1874). It was first described based on specimens from the deposit in Stassfurt, Saxony-Anhalt, Germany, which became a historically important site for its extraction. ## Uses Carnallite is one of the world's most important sources of potassium, primarily used in the production of artificial fertilizers. It is also a significant source of magnesium, which is extracted on an industrial scale. To a lesser extent, it is used in road salt production and in the chemical industry. Due to its physical properties, it is not used in jewelry.

Properties

Mohs hardness
2.5
Luster
Greasy
Streak
White
Density
1.602
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Key diagnostic features of carnallite are its low hardness (2.5 on the Mohs scale), low density, greasy luster on fresh surfaces, and strong hygroscopicity – a specimen left in the air becomes moist and sticky. Its bitter, salty taste is also characteristic. It does not exhibit fluorescence under UV light. ## Distinguishing from Similar Minerals Carnallite can be confused with other evaporites, such as halite (rock salt) or sylvite. Halite has a salty taste (not bitter) and crystallizes in cubic forms. Sylvite also has a bitter-salty taste but is slightly harder and less hygroscopic than carnallite. Kainite is another similar mineral, but it is harder and denser. ## Crystal Forms Well-formed crystals are rare. If they occur, they take tabular or stout prismatic forms, often with a pseudohexagonal outline, which is a result of twinning. Typically, carnallite forms massive, granular, or fibrous aggregates, often intergrown with halite, sylvite, and other salts.

Geological environment

## Genesis Carnallite is a typical evaporite mineral. It forms as a result of water evaporation in closed or restricted marine basins (lagoons) or saline lakes in arid and hot climates. It crystallizes as one of the last minerals in the evaporitic sequence, after the precipitation of calcite, gypsum, anhydrite, and halite. It occurs as layers or lenses within massive rock salt deposits. ## Mineral Associations It most commonly co-occurs with other salt minerals, such as halite, sylvite, anhydrite, kainite, kieserite, and polyhalite. These minerals together form characteristic rock assemblages known as potassium-magnesium salts or complex salts. ## Localities The most historically and economically important deposits are in Germany (Stassfurt, Hanover). Large deposits are also exploited in Russia (Urals, Perm region), Spain (Suria, Cardona), USA (Permian Basin in New Mexico and Texas), Canada (Saskatchewan), China (Qaidam Basin), and in Israel and Jordan (Dead Sea). In Poland, it occurs in salt deposits in Kujawy (Inowrocław and Kłodawa region), but it is not of economic importance there.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are rare, well-formed, transparent crystals, especially those with unusual coloration (blue, violet). Specimens showing distinct pseudohexagonal twinning are also valued. Aesthetic compositions of carnallite with other salt minerals, such as red, granular sylvite or colorless halite, are also attractive. Due to storage difficulties, specimens preserved in ideal condition are rare. ## Popular Localities Classic specimens, prized by collectors, come from historical mines in the Stassfurt region of Germany. Interesting specimens, including fibrous forms, are also found in mines in New Mexico (USA). Specimens from the Dead Sea are also sometimes found in collections.

Care and storage

## Cleaning Carnallite is extremely sensitive to water and moisture. It must absolutely not be cleaned with water or any water-based liquids, as this will cause it to dissolve. Cleaning should be limited to very gently removing dust with a soft, completely dry brush or compressed air from a safe distance. ## What to Avoid Avoid contact with water, water vapor, and high humidity. The mineral is hygroscopic and will absorb moisture from its surroundings, leading to its "melting" and disintegration. Protect it from all chemicals. It is very soft and brittle, so avoid impacts, drops, and scratches. ## Storage Carnallite specimens must be stored in conditions of controlled, low humidity. The best solution is to place them in an airtight container (e.g., a plastic box with a seal) along with a moisture-absorbing agent, such as silica gel. Avoid exposing it to open air, especially on humid days.

External references

Sources

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