Magnesium Selenite, MgSeO₃

Cubic crystals of MgSeO₃.6H₂O separate on adding sodium carbonate solution to a solution of a magnesium salt and selenious acid. It is insoluble in water, soluble in acids (including selenious acid), forms a monohydrate at 100° C., becomes anhydrous between 150° C. and 200° C., and leaves a residue of magnesia at higher temperatures. When treated with water under pressure at 150° C. it transforms into a rhombic dihydrate, which has similar properties to the hexahydrate. A heptahydrate has been precipitated from selenite and magnesium-salt solutions by ammonia, and Berzelius described a trihydrate from the neutralisation of selenious acid with magnesium carbonate.

The acid salts MgSeO₃.H₂SeO₃.3H₂O and MgSeO₃.2H₂SeO₃.3H₂O were obtained by evaporating solutions containing MgO and SeO₂ in suitable proportions, and rhombohedral MgO.2SeO₂ by heating a solution of the neutral salt in selenious acid under pressure at 200° C. The last is also insoluble in water and soluble in acids.