Potassium Calcium Magnesium Sulphates

Polyhalite, KSO₄. 2CaSO₄.MgSO₄.2H₂O, is a characteristic member of the Stassfurt deposits. By boiling gypsum with suitably concentrated solutions of other two sulphates, the series of salts represented by the general formula M2SO₄.2CaSO₄.M'SO₄.2H₂O can be prepared: M' being copper, cadmium, nickel, or zinc, and M being potassium or ammonium. Polyhalite itself results when gypsum is boiled with a concentrated solution of magnesium sulphate containing about 4 per cent, of potassium sulphate. Syngenite, KSO₄.CaSO₄.H₂O, is converted into polyhalite by warming at 56° C. with a solution containing sodium, potassium, and magnesium chlorides, and magnesium sulphate, or with crystalline magnesium sulphate and chloride.

Polyhalite crystallises in the monoclinic system, with a density of 2.77, and hot water dissolves away the sulphates of magnesium and potassium from it.

Krugite, KSO₄.4CaSO₄.MgSO₄.2H₂O, occurring also at Stassfurt, could not be obtained by the interaction of polyhalite and gypsum, though crystals, apparently orthorhombic, were obtained by the prolonged heating of gypsum with a solution containing magnesium chloride and potassium sulphate.

Krugite is crystalline, its density is 2.801, hot water acts on it to produce gypsum, and cold water leaves the insoluble salt KSO₄.CaSO₄.H₂O.