Polyhalite
Polyhalite | |
---|---|
Museum specimen of polyhalite and anhydrite | |
General | |
Category | Sulfate mineral |
Formula (repeating unit) | K2Ca2Mg(SO4)4·2H2O |
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 |
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 |
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 Salzberg, 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.