Double Sulphates of Magnesium

The following have been described: (Mg,Cu)SO₄.Aq., MgSO₄.ZnSO₄.14H₂O and MgSO₄.ZnSO₄.10H₂O, 5MgSO₄.3ZnSO₄.56H₂O, MgSO₄.TlSO₄.6H₂O, MgSO₄.UO₂SO₄.5H₂O, MgSO₄.2MnSO₄.15H₂O (mineral fauserite), MgSO₄.NiSO₄.28H₂O, 2KSO₄.MgSO₄.ZnSO₄.12H₂O and 2KSO₄.MgSO₄.NiSO₄.12H₂O, MgSO₄.ZnSO₄.MnSO₄.21H₂O.

Kainite occurs in the Stassfurt beds, and at Kalusz in Eastern Galicia. Meyerhoffer, from its polytherms (equilibrium relationships at different temperatures), concluded that its formula is MgSO₄.KCl.3H₂O, not MgSO₄.KSO₄.MgCl₂.6H₂O, and that its temperatures of formation lie between 76° C. and 85° C. In the presence of sodium chloride it cannot exist above 83° C. An interaction between carnallite and magnesium sulphate may account for its association with carnallite at Stassfurt.

It is monoclinic, but is usually found in massive granular condition with a reddish, grey, or white colour. Its hardness is about 2.5 and its density about 2.1. It does not deliquesce, and its potash content makes it a valuable and widely used fertiliser.

One hundred parts of water dissolve 79.56 parts at 18° C. Schonite, KSO₄.MgSO₄.6H₂O, crystallises first from the saturated solution, and this and other reactions have been used in working up kainite for potassium and magnesium salts.