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{{Short description|Evaporite mineral}}
{{Distinguish|Carnotite}}
{{Infobox mineral
{{Infobox mineral
| name = Carnallite
| name = Carnallite
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| boxwidth =
| boxwidth =
| boxbgcolor =
| boxbgcolor =
| image = Carnalita.jpg
| image = Carnallit cristalls.jpg
| imagesize =
| imagesize = 256px
| alt =
| alt =
| caption =
| caption = Сarnallite crystals
| formula = KMgCl<sub>3</sub>·6(H<sub>2</sub>O)
| formula = KCl.MgCl<sub>2</sub>·6(H<sub>2</sub>O)
| IMAsymbol=Cna<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
⚫
| strunz = 03.BA.10
| molweight = 277.85 g/mol
⚫
| strunz = 3.BA.10
| dana =
| dana =
| symmetry =
| system = [[Orthorhombic]]
| class = Dipyramidal (mmm) <br/>[[H-M symbol]]: (2/m 2/m 2/m)
| Z =
| molweight = 277.85
| symmetry = ''Pnna''
| color = Blue, colorless, yellow, white, red
| color = Blue, colorless, yellow, white, red
| habit = Fibrous
| habit = Fibrous
| system = [[Orthorhombic]], (2/m 2/m 2/m), [[space group]]: Pcna
| twinning = By pressure, polysynthetic twin lamellae can be developed
| twinning = By pressure, polysynthetic twin lamellae can be developed
| cleavage = None
| cleavage = None
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| diaphaneity = Transparent to translucent
| diaphaneity = Transparent to translucent
| gravity = 1.6
| gravity = 1.6
| density = 1.598g/cm3
| density = 1.598 g/cm<sup>3</sup>
| polish =
| polish =
| opticalprop = Biaxial (+)
| opticalprop = Biaxial (+)
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| other =
| other =
| alteration =
| alteration =
| references = <ref name=Webmin>[http://www.webmineral.com/data/Carnallite.shtml Webmineral data]</ref><ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/carnallite.pdf Handbook of Mineralogy]</ref><ref name=mindat>[http://www.mindat.org/min-906.html Carnallite on Mindat]</ref>
| references = <ref>[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Carnallite Mineralienatlas]</ref><ref name=Webmin>[http://www.webmineral.com/data/Carnallite.shtml Webmineral data]</ref><ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/carnallite.pdf Handbook of Mineralogy]</ref><ref name=mindat>[http://www.mindat.org/min-906.html Carnallite on Mindat]</ref>
}}
}}
'''Carnallite''' is an [[evaporite]] mineral, a hydrated [[potassium]] [[magnesium]] [[chloride]] with formula: [[potassium|K]][[magnesium|Mg]][[chlorine|Cl]]<sub>3</sub>·6([[water|H<sub>2</sub>O]]). 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''' (also '''carnalite''') is an [[evaporite]] mineral, a hydrated [[potassium]] [[magnesium]] [[chloride]] with formula KCl.MgCl<sub>2</sub>·6(H<sub>2</sub>O). 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]].<ref name=mindat/>
Carnallite occurs with a sequence of potassium and magnesium evaporite minerals: [[sylvite]], [[kainite]], [[picromerite]], [[polyhalite]], and [[kieserite]]. Carnallite is an uncommon double chloride mineral that only forms under 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]].<ref name=mindat/>


Carnallite was first described in 1856 from its type location of Stassfurt Deposit, [[Saxony-Anhalt]], [[Germany]]. It was named for the [[Prussia]]n mining [[engineer]], [[Rudolf von Carnall]] (1804–1874).<ref name=mindat/>
Carnallite was first described in 1856 from its type location of Stassfurt Deposit, [[Saxony-Anhalt]], [[Germany]]. It was named for the [[Prussia]]n mining [[engineer]] [[Rudolf von Carnall]] (1804–1874).<ref name=mindat/>
[[Image:Carnalit.jpg|thumb|Carnallite from [[Russia]]]]
[[Image:Carnalit.jpg|thumb|Carnallite from [[Russia]]]]


==Background Information==
==Background==
[[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.<ref name=Bragg>Bragg, L., and G. F. Claringbull. (1965) Crystal Structure of minerals. G. Bell and Sons, Ltd., London.</ref> This means that the dominant large ions are of the Cl<sup>-</sup>, Br<sup>-</sup>, F<sup>-</sup>, and I<sup>-</sup> with a -1 charge. These are easily polarized.<ref name=Klein>Klein, Cornelis, B. Dutrow (2007) Manual of Mineral Science, 23rd ed.John Wiley and Sons</ref><ref name=Bragg/> 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.<ref name=Bragg/> This allows carnallite to be in the same category.
[[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.<ref name=Bragg>Bragg, L., and G. F. Claringbull. (1965) Crystal Structure of minerals. G. Bell and Sons, Ltd., London.</ref> This means that the dominant large ions are the Cl<sup>−</sup>, Br<sup>−</sup>, F<sup>−</sup>, or I<sup>−</sup>. These are easily polarized.<ref name=Bragg/><ref name=Klein>Klein, Cornelis, B. Dutrow (2007) Manual of Mineral Science, 23rd ed.John Wiley and Sons</ref> 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 with the formula KCl. Sylvite precipitates first from mixed solutions of K<sup>+</sup>, Mg<sup>2+</sup> and Cl<sup>−</sup>, leaving a brine enriched in magnesium from which the mixed halide carnallite then precipitates.<ref name=Bragg/>


==Compostion==
==Composition==
Carnallite’s chemical formula is [[potassium|K]][[magnesium|Mg]][[chlorine|Cl]]<sub>3</sub>·6([[water|H<sub>2</sub>O]]). Synthetic Carnallite crystal specimens can be reproduced by 1.5 mole percentage of KCl and 9.8 moles of MgCl2 6 H2O and by slow [[crystallization]] at 25 degrees Celsius<ref name=schlemper>Schlemper, E. O., P. K. Gupta, and Tibor Zoltai. (1985) Refinement of the Structure of Carnallite, Mg(H2O)6KCL3. American Mineralogist 70,1309-1313.</ref>. The [[density]] calculated was 1.587 g/cm3<ref name=schlemper/>. This measurement is fairly close to the measured density 1.602g/cm3; and, it is almost accurate with the calculated density of 1.598g/cm3<ref name=schlemper/>. Carnallite can also be replicated by the combination of hydrated magnesium halide and alkali halides with same anions produced by grinding<ref name=shoval>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
Carnallite's chemical formula is [[potassium|K]][[magnesium|Mg]][[chlorine|Cl]]<sub>3</sub>·6([[water|H<sub>2</sub>O]]). Synthetic carnallite crystal specimens can be produced from 1.5&nbsp;mole percent KCl and 98.5&nbsp;mole percent MgCl<sub>2</sub>·6H<sub>2</sub>O by slow [[crystallization]] at 25&nbsp;°C.<ref name=schlemper>Schlemper, E. O., P. K. Gupta, and Tibor Zoltai. (1985) Refinement of the Structure of Carnallite, Mg(H2O)6KCL3. American Mineralogist 70,1309–1313.</ref> Its density is 1.602&nbsp;g/cm<sup>3</sup>.<ref name=schlemper/> Carnallite can also be produced by grinding the combination of hydrated magnesium chloride and potassium chloride.<ref name=shoval>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</ref>
</ref>


==Structure==
==Structure==
The carnallite structure exhibits corner- and face-sharing. There is a network of KCl<sub>6</sub> octahedra, with two-thirds of them sharing faces.<ref name=schlemper/> Mg(H<sub>2</sub>O)<sub>6</sub> octahedra occupy the open spaces within the KCl octahedra. The interatomic distance between Mg and H<sub>2</sub>O ranges from 0.204 to 0.209&nbsp;nm,<ref name="schlemper" /> with an average is 0.2045&nbsp;nm.<ref name="schlemper" /> The interatomic distance between K and Cl ranges 0.317 to 0.331&nbsp;nm.,<ref name=schlemper/> with an average of 0.324&nbsp;nm.<ref name=schlemper/> The resulting structure has a calculated density of 1.587&nbsp;g/cm<sup>3</sup>, in good agreement with the measured value of 1.602&nbsp;g/cm<sup>3</sup>.<ref name=schlemper/>
⚫
Carnallite has corner and face-sharing. Carnallite minerals have 2/3 of KCl6 [[octahedral face sharing]] networks.<ref name=schlemper/> 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]].<ref name=Klein/> 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]].<ref name=schlemper/> Magnesium has a two + charge. Chlorine has a negative one charge. The five chlorine anions are each coordinated to two potassium cations as well as four water molecules.<ref name=schlemper/> This means that the chlorine anions receive 1/6 positive charge from each of the two potassium molecules. 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.<ref name=schlemper/><ref name=Klein/>

The [[interatomic distance]] range of Mg-H20 is .204 nm - .209 nm.<ref name=schlemper/> The Mg-H20 average is .2045 nm.<ref name=schlemper/> The K-Cl interatomic distance range is .317 nm - .331 nm.<ref name=schlemper/> .324nm rounded is the interatomic distance average of K-Cl.<ref name=schlemper/>
⚫
Face-sharing creates more chance of instability, according to the third of [[Pauling's rules]].<ref name=Klein/> 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.<ref name=schlemper/> The five chloride anions are each coordinated to two potassium cations as well as four water molecules.<ref name=schlemper/> 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.<ref name=Klein/><ref name=schlemper/>


==Physical properties==
==Physical properties==
Carnallite’s [[refractive indices]] range from 1.467 -1.494.<ref name=mottana>Mottana, Annibale, R. Crespi, and G. Liborio. (1978) Rocks and Minerals. Simon and Schuster. NY.</ref><ref name=klein/> Carnallite may be red due to the possible [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>) inclusions<ref name=mottana/>. The fragmented shards of iron oxide produce red tints in the thin [[laminae]] of Hematite<ref name=mottana/>. 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.<ref name=mottana/>
Carnallite's [[refractive index]] ranges from 1.467 to 1.494.<ref name=Klein/><ref name=mottana>Mottana, Annibale, R. Crespi, and G. Liborio. (1978) Rocks and Minerals. Simon and Schuster. NY.</ref> Carnallite may be red as a result of [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>) inclusions.<ref name=mottana/> The fragmented shards of iron oxide produce red tints in the thin [[Lamination (geology)|laminae]] of hematite.<ref name=mottana/> Carnallite is also [[deliquescent]] in high humidity. This implies that it is also extremely soluble in water.<ref name=mottana/> Individual crystals are pseudo-hexagonal and [[tabular habit|tabular]] but are extremely rarely seen.<ref name=blatt>Blatt, H. (1992) Sedimentary Petrology, 2nd ed. W.H. Freeman and Co., San Francisco.</ref> 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.<ref name=blatt/> Carnallite may not only be [[fluorescent]] but is capable of being [[phosphorescent]].<ref name=blatt/> The potassium that carnallite contains fuses easily within a flame, creating a violet color.<ref name=blatt/>


⚫
==Geologic occurrence==
Blatt<ref name=blatt>Blatt, H. (1992) Sedimentary Petrology, 2nd ed. W.H. Freeman and Co., San Fransisco.</ref> 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.<ref name=blatt/> Carnallite can not only be [[fluorescent]] but is capable of being [[phosphorescent]].<ref name=blatt/> Phosphorescent is when the luminescence continues even after the energy causing the exciting rays is cut off.<ref name=Klein/> The potassium Carnallite contains fuses easily creates a violet color within a flame.<ref name=blatt/>
⚫
Mineral associations based on some physical properties include, but not limited to, [[halite]], [[anhydrite]], [[Dolomite (mineral)|dolomite]], [[gypsum]], kainite, kieserite, polyhalite, and sylvite.<ref name=Klein/><ref name=anthony>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.</ref><ref name="economic">Phosphate, potash, and sulfur- A special issue. (1979) Economic Geology 74, 191–493.</ref>


⚫
Carnallite minerals are mineral sediments known as [[evaporite]]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.<ref name=Smetannikov>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</ref> Closer to 10 percent sylvite followed by Carnallite form.<ref name=Smetannikov/>
⚫
==Geologic Occurrence==

⚫
Mineral associations based on some physical properties but not limited to, are; halite, [[anhydrite]], dolomite, gypsum, [[kainite]], [[kieserite]], [[polyhalite]], and sylvite is the most common associated mineral<ref name=anthony>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.</ref><ref name=klein/><ref name=Economic>Phosphate, potash, and sulfur- A special issue. (1979) Economic Geology 74, 191-493.</ref>.
Carnallite is mostly found in saline marine deposits,<ref name=blatt/> although beds exist in the [[endorheic basin|endorheic]] [[Qaidam Basin]] of [[China]]'s [[Qinghai Province]] near [[Dabusun Nor (Qinghai)|Dabusun Nor]].<ref>{{citation |last=Garrett |first=Donald Everett |url=https://books.google.com/books?id=_MIjCQAAQBAJ |title=Potash: Deposits, Processing, Properties, and Uses |date=1996 |location=London |publisher=Chapman & Hall |page=177 |isbn=9789400915459 }}.</ref>
⚫
Carnallite is found in saline marine deposits<ref name=blatt/>. 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<ref name=Smetannikov>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</ref>. Closer to 10 percent sylvite followed by Carnallite form<ref name=Smetannikov/>.


==Uses==
==Uses==
Carnallite is mostly used in fertilizers. It is an important source of [[potash]]<ref name=economic>Phosphate, potash, and sulfur- A special issue. (1979) Economic Geology 74, 191-493.</ref>. Only Sylvite outranks Carnallite’s importance in potash<ref name=Economic/>. They are difficult to form since Sylvite and Carnallite are some of the last evaporites to form<ref name=economic/>. Soluble Potassium salts are the main sources for fertilizer. This is because insoluble [[potassium feldspars]] are difficult to separate<ref name=economic/>. Carnallite may be a low source of magnesium worldwide; however, it is Russia’s main source<ref name=economic/>.
Carnallite is mostly used in fertilizers. It is an important source of [[potash]].<ref name="economic"/> Only [[sylvite]] outranks carnallite's importance in potash production.<ref name="economic"/> Both are uncommon because they are some of the last evaporites to form.<ref name=economic/> Soluble potassium salts are the main sources for fertilizer. This is because the potassium is difficult to separate from insoluble [[potassium feldspar]].<ref name=economic/> Carnallite is a minor source of magnesium worldwide; however, it is Russia's main source.<ref name=economic/>



==See also==
==See also==
Line 83: Line 86:


==References==
==References==
{{reflist}}
{{Reflist}}




{{Authority control}}


[[Category:Potassium minerals]]
[[Category:Potassium minerals]]
[[Category:Magnesium minerals]]
[[Category:Magnesium minerals]]
[[Category:Halide minerals]]
[[Category:Chloride minerals]]
[[Category:Orthorhombic minerals]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 52]]

[[Category:Evaporite]]
{{Halide-mineral-stub}}
[[Category:Potash]]

[[ca:Carnalita]]
[[de:Carnallit]]
[[es:Carnalita]]
[[fa:کارنالیت]]
[[it:Carnallite]]
[[he:קרנליט]]
[[kk:Карналлит]]
[[nl:Carnalliet]]
[[ja:カーナライト]]
[[pl:Karnalit]]
[[pt:Carnallita]]
[[ru:Карналлит]]
[[sv:Karnallit]]
[[uk:Карналіт]]

Latest revision as of 19:40, 16 January 2024

Not to be confused with Carnotite.
Carnallite
Сarnallite crystals
General
CategoryHalide mineral
Formula
(repeating unit)		KCl.MgCl2·6(H2O)
IMA symbolCna[1]
Strunz classification3.BA.10
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnna
Identification
Formula mass277.85 g/mol
ColorBlue, colorless, yellow, white, red
Crystal habitFibrous
TwinningBy pressure, polysynthetic twin lamellae can be developed
CleavageNone
FractureConchoidal
Mohs scale hardness2.5
LusterGreasy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity1.6
Density1.598 g/cm3
Optical propertiesBiaxial (+)
Refractive indexnα = 1.467
nβ = 1.476
nγ = 1.494
Birefringence0.0270
2V angle70
References[2][3][4][5]

Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KCl.MgCl2·6(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 an uncommon double chloride mineral that only forms under 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.[5]

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).[5]

Carnallite from Russia

Background

[edit]

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.[6] This means that the dominant large ions are the Cl−, Br−, F−, or I−. These are easily polarized.[6][7] 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 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.[6]

Composition

[edit]

Carnallite's chemical formula is KMgCl3·6(H2O). Synthetic carnallite crystal specimens can be produced from 1.5 mole percent KCl and 98.5 mole percent MgCl2·6H2O by slow crystallization at 25 °C.[8] Its density is 1.602 g/cm3.[8] Carnallite can also be produced by grinding the combination of hydrated magnesium chloride and potassium chloride.[9]

Structure

[edit]

The carnallite structure exhibits corner- and face-sharing. There is a network of KCl6 octahedra, with two-thirds of them sharing faces.[8] 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,[8] with an average is 0.2045 nm.[8] The interatomic distance between K and Cl ranges 0.317 to 0.331 nm.,[8] with an average of 0.324 nm.[8] The resulting structure has a calculated density of 1.587 g/cm3, in good agreement with the measured value of 1.602 g/cm3.[8]

Face-sharing creates more chance of instability, according to the third of Pauling's rules.[7] 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.[8] The five chloride anions are each coordinated to two potassium cations as well as four water molecules.[8] 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.[7][8]

Physical properties

[edit]

Carnallite's refractive index ranges from 1.467 to 1.494.[7][10] Carnallite may be red as a result of hematite (Fe2O3) inclusions.[10] The fragmented shards of iron oxide produce red tints in the thin laminae of hematite.[10] Carnallite is also deliquescent in high humidity. This implies that it is also extremely soluble in water.[10] Individual crystals are pseudo-hexagonal and tabular but are extremely rarely seen.[11] 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.[11] Carnallite may not only be fluorescent but is capable of being phosphorescent.[11] The potassium that carnallite contains fuses easily within a flame, creating a violet color.[11]

Geologic occurrence

[edit]

Mineral associations based on some physical properties include, but not limited to, halite, anhydrite, dolomite, gypsum, kainite, kieserite, polyhalite, and sylvite.[7][12][13]

Carnallite minerals are mineral sediments known as evaporites. 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.[14] Closer to 10 percent sylvite followed by Carnallite form.[14]

Carnallite is mostly found in saline marine deposits,[11] although beds exist in the endorheic Qaidam Basin of China's Qinghai Province near Dabusun Nor.[15]

Uses

[edit]

Carnallite is mostly used in fertilizers. It is an important source of potash.[13] Only sylvite outranks carnallite's importance in potash production.[13] Both are uncommon because they are some of the last evaporites to form.[13] Soluble potassium salts are the main sources for fertilizer. This is because the potassium is difficult to separate from insoluble potassium feldspar.[13] Carnallite is a minor source of magnesium worldwide; however, it is Russia's main source.[13]

See also

[edit]

References

[edit]
  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Mineralienatlas
  3. ^ Webmineral data
  4. ^ Handbook of Mineralogy
  5. ^ a b c Carnallite on Mindat
  6. ^ a b c Bragg, L., and G. F. Claringbull. (1965) Crystal Structure of minerals. G. Bell and Sons, Ltd., London.
  7. ^ a b c d e Klein, Cornelis, B. Dutrow (2007) Manual of Mineral Science, 23rd ed.John Wiley and Sons
  8. ^ a b c d e f g h i j k Schlemper, E. O., P. K. Gupta, and Tibor Zoltai. (1985) Refinement of the Structure of Carnallite, Mg(H2O)6KCL3. American Mineralogist 70,1309–1313.
  9. ^ 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
  10. ^ a b c d Mottana, Annibale, R. Crespi, and G. Liborio. (1978) Rocks and Minerals. Simon and Schuster. NY.
  11. ^ a b c d e Blatt, H. (1992) Sedimentary Petrology, 2nd ed. W.H. Freeman and Co., San Francisco.
  12. ^ 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.
  13. ^ a b c d e f Phosphate, potash, and sulfur- A special issue. (1979) Economic Geology 74, 191–493.
  14. ^ a b 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
  15. ^ Garrett, Donald Everett (1996), Potash: Deposits, Processing, Properties, and Uses, London: Chapman & Hall, p. 177, ISBN 9789400915459.
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