Polyhalite | |
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![]() 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.