a mineral, a hydrated sulfate of potassium, calcium, and magnesium with composition K₂Ca₂Mg[SO₄]₄ · 2H₂O. It contains 15.62 percent K₂O, 18.60 percent CaO, 6.69 percent MgO, 53.11 percent SO₃, and 5.98 percent H₂O. Polyhalite crystallizes in the triclinic system, forming granular aggregates and, more rarely, fibrous or columnar aggregates. Tabular and elongated crystals with distinct cleavage are rare. Polyhalite is colorless, gray, pink, or brick-red. It has a hardness of 2.5–3.5 on Mohs’ scale and a density of 2,720–2,780 kg/m^3· It decomposes upon the action of water to precipitate gypsum.

Polyhalite is widely distributed in commercial salt deposits, where it is most often associated with rock salt and anhydrite, as well as with sylvite, carnallite, and kieserite. It may be used in the preparation of potassium fertilizers.