Magnesium Orthophosphate, Mg₃(PO₄)₂

Magnesium Orthophosphate occurs as bobierrite, Mg₃(PO₄)₂. 8H₂O, in wagnerite, 2Mg₃(PO₄)₂.MgF₂, and in the parts and products of animals or plants - in bones, for example, and in guano.

When a magnesium salt is precipitated cold by trisodium phosphate, colloidal, gelatinous magnesium orthophosphate first forms which slowly becomes crystalline. Mg₃(PO₄)₂.22H₂O separates at 10° C. from mixed solutions of magnesium sulphate, disodium hydrogen phosphate, and sodium bicarbonate: the precipitate from more concentrated solutions is amorphous, but crystals of density 1.640 separate at greater dilutions. A crystalline octahydrate separates from still more dilute solutions at 20°-25° C., and larger monoclinic crystals with a density of 2.195, resembling bobierrite, result when dilute ammonia is dropped into a solution containing magnesium chloride, disodium hydrogen phosphate, and hydrochloric acid. It loses water over sulphuric acid or at 100° C., and becomes anhydrous at a red heat.

The heptahydrate occurs in guano, and has been prepared in small hexagonal crystals by boiling acid magnesium phosphate with magnesium carbonate.

The pentahydrate has been prepared by the action of water at 120° C. on MgHPO₄, and is precipitated as a white, amorphous, tasteless powder when trisodium phosphate is boiled with excess of magnesium sulphate.

One part of freshly precipitated magnesium orthophosphate dissolves in about 5000 parts of water and in about 10,000 parts after ignition. The solubility is increased by sodium chloride and some other salts. The precipitate dissolves readily in mineral acids, very much less readily in acetic acid, and when dehydrated by calcination becomes phosphorescent.

Natural bobierrite crystallises in the monoclinic system with a hardness of 2.5 and a density of 2.43. Wagnerite is monoclinic, and has a hardness of 5-5.5; its density is 3.06.