Potassium Magnesium Sulphates

Schdnite, or picromerite, KSO₄.MgSO₄.6H₂O, occurs in the Stassfurt deposits, and crystallises in transparent monoclinic prisms from a solution of the mixed salts in equimolecular proportions at ordinary temperatures. Their density at 20°/4° is 2.0277. Since schonite is produced at -3° C. according to the equation

KSO₄+MgSO₄.7H₂O ⇔ KSO₄.MgSO₄.6H₂O+H₂O,

and since MgSO₄.12H₂O is the stable hydrate between -3.9° C. and 1.8° C., the system should become labile at the latter temperature.

One hundred parts of water dissolve 25 parts at 25° C. and 59.8 parts at 75° C.

Schonite passes into leonite, KSO₄.MgSO₄.4H₂O, at 47.5° C., and at lower temperatures in the presence of sodium chloride. Nitric acid effects the reverse change. It crystallises in tables, and has been prepared by evaporating a solution of schonite and sodium chloride at 60° C. Leonite occurs naturally at Leopoldshall, near Stassfurt, in thick, tabular crystals which usually have a yellowish tinge and are soluble in water.

Leonite in solution is transformed into langbeinite, KSO₄.2MgSO₄, at 89° C., and at lower temperatures in the presence of other salts. Langbeinite has been prepared by treating schonite with superheated steam, or kainite with water under pressure, or kainite with magnesium chloride solution from 120°-145° C., and by melting its component salts together. It occurs in the Prussian salt deposits and elsewhere in isometric crystals. The mineral has a glassy lustre, a conchoidal fracture, a hardness of 3.4, and a density of 2.83. It is hygroscopic and slowly decomposed by water. It occurs rarely in cubical crystals, and the artificial crystals are usually octahedral. It melts at 930° C. and shows simple refraction (D²⁵ = 2.629).

Artificial langbeinite, produced by fusing MgSO₄ and KSO₄ in molecular proportions, is converted by exposure to air into schonite and epsomite -

KSO₄.2MgSO₄+13H₂O = KSO₄.MgSO₄.6H₂O+MgSO₄.7H₂O.
[KSO₄.MgSO₄] +Aq. = KSO₄.MgSO₄.Aq. + 10.60 Cal.
[KSO₄.MgSO₄.6H₂O] + Aq. = KSO₄.MgSO₄.Aq. -10.024 Cal.
[KSO₄.MgSO₄.4H₂O] + Aq. = KSO₄.MgSO₄.Aq. -5.16 Cal.