Carnallite

Carnallite
Сarnallite crystals
General
Category	Halide mineral
Formula (repeating unit)	KCl.MgCl2·6(H2O)
Strunz classification	3.BA.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnna
Identification
Formula mass	277.85 g/mol
Color	Blue, colorless, yellow, white, red
Crystal habit	Fibrous
Twinning	By pressure, polysynthetic twin lamellae can be developed
Cleavage	None
Fracture	Conchoidal
Mohs scale hardness	2.5
Luster	Greasy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	1.6
Density	1.598 g/cm3
Optical properties	Biaxial (+)
Refractive index	nα = 1.467 nβ = 1.476 nγ = 1.494
Birefringence	0.0270
2V angle	70
References

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 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.