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Carnallite (also carnalite) is an
evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
mineral, a hydrated
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
(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 Kainite ( or ) (KMg(SO4)Cl·3H2O) is an evaporite mineral in the class of "Sulfates (selenates, etc.) with additional anions, with H2O" according to the Nickel–Strunz classification. It is a hydrated potassium-magnesium sulfate-chloride, natura ...
,
picromerite Picromerite (synonyms: schoenite, schönite) is a mineral from the class of hydrous sulfates lacking additional anions, and containing medium to large cations according to the Nickel–Strunz classification. Etymology The name comes from the Gr ...
,
polyhalite Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: . Polyhalite crystallizes in the triclinic system, although crystals are very rare. The normal habit is massive to fibrous. It is typically c ...
, and
kieserite Kieserite, or magnesium sulfate monohydrate, is a hydrous magnesium sulfate mineral with formula (MgSO4·H2O). It has a vitreous luster and it is colorless, grayish-white or yellowish. Its hardness is 3.5 and crystallizes in the monoclinic cry ...
. Carnallite is an uncommon double chloride mineral that only forms under specific environmental conditions in an evaporating sea or
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
. It is mined for both potassium and magnesium and occurs in the evaporite deposits of
Carlsbad, New Mexico Carlsbad ( ) is a city in and the county seat of Eddy County, New Mexico, Eddy County, New Mexico, United States. As of the 2020 United States Census, 2020 census, the city population was 32,238. Carlsbad is centered at the intersection of U.S. R ...
; the
Paradox Basin The Paradox Basin is an asymmetric foreland basin located mostly in southeast Utah and southwest Colorado, but extending into northeast Arizona and northwest New Mexico. The basin is a large elongate northwest to southeast oriented depression for ...
in
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...
and
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
; Stassfurt,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
; the Perm Basin,
Russia Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
; and the
Williston Basin The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North Dakota, South Dakota, southern Saskatchewan, and south-western Manitoba that is known for its rich deposits of petroleum and potash. The basin is a ...
in
Saskatchewan, Canada Saskatchewan ( ; ) is a province in western Canada, bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and on the south by the U.S. states of Montana and North Dak ...
. These deposits date from the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
through the
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
Periods. In contrast, both
Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
and
Jordan Jordan ( ar, الأردن; tr. ' ), officially the Hashemite Kingdom of Jordan,; tr. ' is a country in Western Asia. It is situated at the crossroads of Asia, Africa, and Europe, within the Levant region, on the East Bank of the Jordan Rive ...
produce
potash Potash () includes various mined and manufactured salts that contain potassium in water-soluble form.
from the
Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...
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 Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in natu ...
from the
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...
. Carnallite was first described in 1856 from its type location of Stassfurt Deposit,
Saxony-Anhalt Saxony-Anhalt (german: Sachsen-Anhalt ; nds, Sassen-Anholt) is a state of Germany, bordering the states of Brandenburg, Saxony, Thuringia and Lower Saxony. It covers an area of and has a population of 2.18 million inhabitants, making it the ...
,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
. It was named for the
Prussia Prussia, , Old Prussian: ''Prūsa'' or ''Prūsija'' was a German state on the southeast coast of the Baltic Sea. It formed the German Empire under Prussian rule when it united the German states in 1871. It was ''de facto'' dissolved by an em ...
n mining
engineer Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the l ...
Rudolf von Carnall (1804–1874).


Background

Halides In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
are
binary compounds In materials chemistry, a binary phase or binary compound is a chemical compound containing two different elements. Some binary phase compounds are molecular, e.g. carbon tetrachloride (CCl4). More typically binary phase refers to extended soli ...
. They are composed of a
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
and a metal ion. The crystal chemistry of halides is characterized by the
electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
of halogen ions.Bragg, L., and G. F. Claringbull. (1965) Crystal Structure of minerals. G. Bell and Sons, Ltd., London. This means that the dominant large ions are the Cl, Br, F, or I. These are easily polarized.Klein, Cornelis, B. Dutrow (2007) Manual of Mineral Science, 23rd ed.John Wiley and Sons The ions combine with similarly large but low valence and weakly polarized cations. The cations are mostly of the
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
group. Sylvite is a binary compound with the formula KCl. Sylvite precipitates first from mixed solutions of K+, Mg2+ and Cl, leaving a brine enriched in magnesium from which the mixed halide carnallite then precipitates.


Composition

Carnallite's chemical formula is K Mg Cl3·6( H2O). Synthetic carnallite crystal specimens can be produced from 1.5 mole percent KCl and 98.5 mole percent MgCl2·6H2O by slow
crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
at 25 °C.Schlemper, E. O., P. K. Gupta, and Tibor Zoltai. (1985) Refinement of the Structure of Carnallite, Mg(H2O)6KCL3. American Mineralogist 70,1309–1313. Its density is 1.602 g/cm3. Carnallite can also be produced by grinding the combination of hydrated magnesium chloride and potassium chloride.Shoval, S., S. Yariv. (1998) Formation of Carnallite Type Double Salts by Grinding Mixtures of Magnesium and Alkali Halides with the Same Anions. Journal of Thermal Analysis 51, 251–263


Structure

The carnallite structure exhibits corner- and face-sharing. There is a network of KCl6 octahedra, with two-thirds of them sharing faces. Mg(H2O)6 octahedra occupy the open spaces within the KCl octahedra. The interatomic distance between Mg and H2O ranges from 0.204 to 0.209 nm, with an average is 0.2045 nm. The interatomic distance between K and Cl ranges 0.317 to 0.331 nm., with an average of 0.324 nm. The resulting structure has a calculated density of 1.587 g/cm3, in good agreement with the measured value of 1.602 g/cm3. Face-sharing creates more chance of instability, according to the third of
Pauling's rules Pauling's rules are five rules published by Linus Pauling in 1929 for predicting and rationalizing the crystal structures of ionic compounds. First rule: the radius ratio rule For typical ionic solids, the cations are smaller than the anions ...
. In carnallite, the water molecules enclose the magnesium ions. This prevents the magnesium and the chloride from interacting directly; instead, the water molecules act as charge transmitters. The five chloride anions are each coordinated to two potassium cations as well as four water molecules. This means that each chloride anion receives 1/6 of a +1 charge from each of the two potassium ions. The chloride also obtains 1/6 of a +1 charge from each the four water molecules. The charges thus total six 1/6 positive charges, which balance the negative charge of the chloride. These two aspects render the rare face-sharing described by the second and third of Pauling's rules acceptable in the carnallite structure.


Physical properties

Carnallite's
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
ranges from 1.467 to 1.494.Mottana, Annibale, R. Crespi, and G. Liborio. (1978) Rocks and Minerals. Simon and Schuster. NY. Carnallite may be red as a result of
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
(Fe2O3) inclusions. The fragmented shards of iron oxide produce red tints in the thin laminae of hematite. Carnallite is also
deliquescent Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
in high humidity. This implies that it is also extremely soluble in water. Individual crystals are pseudo-hexagonal and
tabular Table may refer to: * Table (furniture), a piece of furniture with a flat surface and one or more legs * Table (landform), a flat area of land * Table (information), a data arrangement with rows and columns * Table (database), how the table data ...
but are extremely rarely seen.Blatt, H. (1992) Sedimentary Petrology, 2nd ed. W.H. Freeman and Co., San Francisco. 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 Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...
. Carnallite has a bitter taste. Carnallite may not only be
fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
but is capable of being
phosphorescent Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...
. The potassium that carnallite contains fuses easily within a flame, creating a violet color.


Geologic occurrence

Mineral associations based on some physical properties include, but not limited to,
halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
,
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...
,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
,
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
, kainite, kieserite, polyhalite, and sylvite.Anthony, J. W., R. A. Bideaux, R. A., Bladh, K. W. and M. C. Nichols. (1997) Handbook of Mineralogy. Vol. 3 Halides, hydroxides, oxides. Mineral Data Publications, Tucson, Arizona.Phosphate, potash, and sulfur- A special issue. (1979) Economic Geology 74, 191–493. Carnallite minerals are mineral sediments known as
evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
s. Evaporites are concentrated by evaporation of seawater. The inflow 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.Smetannikov, A. F., (2010) Hydrogen Generation during the Radiolysis of Crystallization water in Carnallite and Possible Consequences of this Process Geochemistry International 49, 971–980 Closer to 10 percent sylvite followed by Carnallite form. Carnallite is mostly found in saline marine deposits, although beds exist in the
endorheic An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but drainage converges instead into lakes ...
Qaidam Basin of
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
's
Qinghai Province Qinghai (; alternately romanized as Tsinghai, Ch'inghai), also known as Kokonor, is a landlocked province in the northwest of the People's Republic of China. It is the fourth largest province of China by area and has the third smallest pop ...
near Dabusun Nor..


Uses

Carnallite is mostly used in fertilizers. It is an important source of
potash Potash () includes various mined and manufactured salts that contain potassium in water-soluble form.
. Only sylvite outranks carnallite's importance in potash production. Both are uncommon because they are some of the last evaporites to form. Soluble potassium salts are the main sources for fertilizer. This is because the potassium is difficult to separate from insoluble
potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium: *Orthoclase (endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock *Microcline, chemically the same as orthoclas ...
. Carnallite is a minor source of magnesium worldwide; however, it is Russia's main source.


See also

*
Evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
*
List of minerals This is a list of minerals for which there are articles on Wikipedia. Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various ''species''. Within a m ...
*
List of minerals named after people This is a list of minerals named after people. The chemical composition follows name. A *Abelsonite: C31H32N4Ni – American physicist Philip Hauge Abelson (1913–2004)alfred *Abswurmbachite: Cu2+Mn3+6O8SiO4 – German mineralogist I ...


References

{{Authority control Potassium minerals Magnesium minerals Halide minerals Orthorhombic minerals Minerals in space group 52 Evaporite Potash