polyhalite

{{infobox mineral | name = Polyhalite | boxwidth = | boxbgcolor = | image = History museum of Truskavets 081.jpg | imagesize = | alt = | caption = Museum specimen of polyhalite and anhydrite | category = Sulfate mineral | formula = | IMAsymbol = Plhl | molweight = | strunz = 7.CC.65 | dana = | system = Triclinic | class = Pinacoidal () (same H-M symbol) | symmetry = F | 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 | colour = | habit = Typically fibrous, foliated, massive; rarely as tabular crystals; pseudo-orthorhombic | twinning = Characteristically polysynthetic on {010}, {100} | cleavage = Perfect on {10}; parting on {010} | fracture = Conchoidal | tenacity = Brittle | mohs = 3.5 | luster = Vitreous to resinous | streak = White | diaphaneity = Transparent | gravity = 2.78 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.546 – 1.548 nβ = 1.558 – 1.562 nγ = 1.567 | birefringence = δ = 0.021 | pleochroism = | 2V = Measured: 60° to 62° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = Soluble in water, with precipitation of gypsum and perhaps syngenite | impurities = | alteration = | other = | references = }} Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: . 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 valuable fertilizer.

Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria. 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.