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For polyhalide, see Polyhalogen ions.
History museum of Truskavets 081.jpg
Museum specimen of polyhalite and anhydrite
Category Sulfate mineral
(repeating unit)
Strunz classification 07.CC.65
Crystal symmetry Triclinic pinacoidal Space group: P1
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
Color Colorless, white, gray; often salmon-pink to brick-red from included iron oxide
Crystal habit Typically fibrous, foliated, massive; rarely as tabular crystals
Crystal system Triclinic (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 [1][2][3]

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 occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad Deposits of New Mexico.

Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria.[1] The name is from the Greek polys hals for many salts.[3]

It has no connection to the sodium halide mineral halite other than that both are evaporite minerals.

Polyhalite is used as a specialty fertilizer since it contains four important nutrients and is low in chlorine.

Crystal structure of polyhalite