Magnesium Sulphates

Na₂SO₄.3MgSO₄, melting with decomposition at 814° C., 3Na₂SO₄.MgSO₄, corresponding to vant-hoffite, and Na₂SO₄.MgSO₄, existing in three modifications and corresponding to anhydrous blodite, are indicated on the freezing-point curve.

Blodite is formed at 22° C. according to the equation

MgSO₄.7H₂O +Na₂SO₄.10H₂O ⇔ Na₂SO₄.MgSO₄.4H₂O + 13H₂O.

The double salt crystallises from the mixed solution between 25° C. and 71° C., and below 22° C. a mixture of Epsom and Glauber's salts crystallises. Blodite is identical with simonite from Hallstadt, and has been called astrakanite from its occurrence in the salt lakes of the Astrakan Steppe. It is associated with carnallite and kainite in the Stassfurt deposits. Monoclinic crystals of blodite occur at Monte Sambuco in Sicily, and isolated crystals, weighing up to 652 grm., have been found in the crust on Soda Lake, California.

Blodite transforms into loewite at 71° C., and at 59° C. in the presence of sodium chloride or of magnesium sulphate. Loewite has the formula 2Na₂SO₄.2MgSO₄.5H₂O, and occurs naturally in the Magdeburg-Hallerstadt salt district.

The double salt Na₂SO₄.2MgSO₄, analogous to langbeinite, apparently cannot exist, at any rate below 90° C. Vanthoffite, 3Na₂SO₄.MgSO₄, occurs naturally in the same district as loewite, and has been prepared by the transformation of blodite at higher temperatures than its transition-point into loewite. Natural vanthoffite has the same D²⁵ = 2.6955 as the artificial, and shows an alteration in refraction at 489° C.

Prismatic crystals of Na₂SO₄.MgSO₄.8H₂O have been reported as a saline efflorescence in Toledo.

Blodite crystallises in the monoclinic system with a hardness of 2.5 and a density of 2.2-2.3; loewite crystallises in the tetragonal system with a density of 2.376 and a hardness of 2.5-3.