Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K2Ca2Mg(SO4)4·2H2O.

It is typically colorless, white to gray, although it may be brick red due to iron oxide inclusions.

[2] It occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad deposits of New Mexico.

[5][4] Despite the similarity in names between polyhalite and halite (the naturally occurring form of table salt), their only connection is that both are evaporite minerals.

[6] In March 2020, the Woodsmith project was taken over by Anglo American plc and construction of two 1,500 m (4,900 ft) shafts to reach the 230 feet (70 m) mineral seam was underway in 2015.