Polyhalite

Polyhalite
Museum specimen of polyhalite and anhydrite
General
Category	Sulfate mineral
Formula (repeating unit)	K2Ca2Mg(SO4)4·2H2O
IMA symbol	Plhl[1]
Strunz classification	7.CC.65
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	F1
Unit cell	a = 6.95 Å, b = 8.88 Å, c = 6.95 Å; α = 104.06°, β = 113.94°, γ = 101.15°; Z = 4
Identification
Color	Colorless, white, gray; often salmon-pink to brick-red from included iron oxide
Crystal habit	Typically fibrous, foliated, massive; rarely as tabular crystals; pseudo-orthorhombic
Twinning	Characteristically polysynthetic on {010}, {100}
Cleavage	Perfect on {101}; parting on {010}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3.5
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.78
Optical properties	Biaxial (−)
Refractive index	nα = 1.546 – 1.548 nβ = 1.558 – 1.562 nγ = 1.567
Birefringence	δ = 0.021
2V angle	Measured: 60° to 62°
Solubility	Soluble in water, with precipitation of gypsum and perhaps syngenite
References	[2][3][4]

Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K₂Ca₂Mg(SO₄)₄·2H₂O. Polyhalite crystallizes in the triclinic system, although crystals are very rare. The normal habit is massive to fibrous. It is typically colorless, white to gray, although it may be brick red due to iron oxide inclusions. It has a Mohs hardness of 3.5 and a specific gravity of 2.8. It is used as a fertilizer.

Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria.^[2] It occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad deposits of New Mexico. It is also present as a 2–3% contaminant of Himalayan salt. The only deposit currently being mined lies under North Yorkshire, UK, extending under the adjacent North Sea.