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Carnallite
Chemical Formula	KMgCl3·6(H2O)
Species	Halides
Crystal System	Orthorhombic
Mohs Scale	2
Specific Gravity	1.598
Color	Blue, colorless, yellow, white, red
Streak	White
Luster	Greasy
Refractive Index	n = 1.465 - 1.466 n = 1.474 - 1.475 n = 1.494 - 1.496
Diaphaneity	Transparent, Translucent
Cleavage	None Observed
Fracture	Conchoidal
Crystal Habit：Fibrous
Geological Setting：Saline marine deposits by action of high potash fluids.

Carnallite is an evaporite mineral, a hydrated potassium magnesium chloride with formula: KMgCl₃·6(H₂O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with rare pseudohexagonal orthorhombic crystals. The mineral is deliquescent (absorbs moisture from the surrounding air) and specimens must be stored in an airtight container.

Carnallite occurs with a sequence of potassium and magnesium evaporite minerals sylvite, kainite, picromerite, polyhalite and kieserite. Carnallite is a somewhat rare double chloride mineral which only forms under a specific environmental conditions in an evaporating sea or sedimentary basin. It is mined for both potassium and magnesium and occurs in the evaporite deposits of Carlsbad, New Mexico; the Paradox Basin in Colorado and Utah; Stassfurt, Germany; the Perm Basin, Russia; and the Williston Basin in Saskatchewan, Canada. These deposits date from the Devonian through the Permian Periods. In contrast, both Israel and Jordan produce potash from the Dead Sea by using evaporation pans to further concentrate the brine until carnallite precipitates, dredging the carnallite from the pans, and processing to remove the magnesium chloride from the potassium chloride.

Carnallite was first described in 1856 from its type location of Stassfurt Deposit, Saxony-Anhalt, Germany. It was named for the Prussian mining engineer, Rudolf von Carnall (1804–1874).

Background Information

Halides are binary compounds. They are composed of a halogen and a metal ion. The crystal chemistry of halides is characterized by the electronegativity of halogen ions. This means that the dominant large ions are of the Cl^-, Br^-, F^-, and I^- with a -1 charge. These are easily polarized. The ions combine with similarly large but low valence and weakly polarized cations. The cations are mostly of the alkali metal group. Sylvite is a binary compound, composed of KCl. Sylvite precipitates forming carnallite in the process. This allows carnallite to be in the same category.

Compostion

Carnallite’s chemical formula is KMgCl₃·6(H₂O). Synthetic carnallite crystal specimens can be reproduced by 1.5 mole percentage of KCl and 9.8 moles of MgCl₂·6H₂O and by slow crystallization at 25 °C. The density calculated was 1.587 g/cm³. This measurement is fairly close to the measured density 1.602 g/cm³; and, it is almost accurate with the calculated density of 1.598 g/cm³. Carnallite can also be replicated by the combination of hydrated magnesium halide and alkali halides with same anions produced by grinding.

Structure

Carnallite has corner and face-sharing. Carnallite minerals have 2/3 of KCl₆ octahedral face sharing networks. Mg(H₂0)₆ octahedra occupy the open spaces within the KCl octahedra. Face sharing creates more chance of instability according to the third rule from Pauling’s Rules. The water molecules enclose the magnesium atoms. This prevents the magnesium and the chlorine from interacting. By being between magnesium and chlorine atoms, the water molecules act as charge transmitters. Magnesium has a 2⁺ charge. Chlorine has a negative one charge. The five chlorine anions are each coordinated to two potassium cations as well as four water molecules. This means that the chlorine anions receive 1/6 positive charge from each of the two potassium ionss. Chlorine also obtains 1/6 positive charge from the four water molecules. The charges total to six 1/6 positive charge that balance the negative charge of chlorine. These two aspects allow the rare face sharing described of the second and third rules of Pauling’s rule acceptable in the carnallite structure. The interatomic distance range of Mg-H₂0 is 0.204 nm - 0.209 nm. The Mg-H₂0 average is 0.2045 nm. The K-Cl interatomic distance range is 0.317 nm - 0.331 nm. 0.324 nm rounded is the interatomic distance average of K-Cl.

Physical properties

Carnallite’s refractive indices range from 1.467 -1.494. Carnallite may be red due to the possible hematite (Fe₂O₃) inclusions. The fragmented shards of iron oxide produce red tints in the thin laminae of hematite. Carnallite is also deliquescent in high humidity. This means that it absorbs the moisture in the air and disintegrates. This also implies that it is extremely soluble in water.

Blatt supplements, individual crystals are pseudo-hexagonal and tabular but are extremely rare to be seen. Field indicators of carnallite are: environment of formation, absence of cleavage, and fracture. Other indicators can be: density, taste, associations to local minerals, and whether it is capable of luminescence. Carnallite has a bitter taste. Carnallite can not only be fluorescent but is capable of being phosphorescent. Phosphorescent is when the luminescence continues even after the energy causing the exciting rays is cut off. The potassium Carnallite contains fuses easily creates a violet color within a flame.

Geologic occurrence

Mineral associations based on some physical properties but not limited to, are; halite, anhydrite, dolomite, gypsum, kainite, kieserite, polyhalite, and sylvite.

Carnallite is found in saline marine deposits. Carnallite minerals are mineral sediments known as evaporites. Evaporites are concentrated by evaporation, in seawater. The intake of water must be below the evaporation or use levels. This creates a prolonged evaporation period. In controlled environment experiments the halides form when 10%- 20% of the original sample of water remains. Closer to 10 percent sylvite followed by Carnallite form.

Uses

Carnallite is mostly used in fertilizers. It is an important source of potash. Only Sylvite outranks Carnallite’s importance in potash. They are difficult to form since sylvite and carnallite are some of the last evaporites to form. Soluble potassium salts are the main sources for fertilizer. This is because insoluble potassium feldspars are difficult to separate. Carnallite may be a low source of magnesium worldwide; however, it is Russia’s main source.