Basic Calcium Phosphates

Basic slag is supposed to contain tetracalcium phosphate, 4CaO.P₂O₅, of melting-point 1630° C. Kroll regarded it as a basic ultraphosphate, 12CaO.3P₂O₅.

By heating tricalcium phosphate to 540° C., oxyapatite, 3Ca₃P₄O₈.CaO, is formed. It crystallises in doubly refracting needles of melting-point 1540° C. and density 2.99. It is also obtained when tetracalcium phosphate is heated for some time at 1000° C., or is treated with boiling water.

According to Bassett, the solid existing in stable equilibrium at 25° C., with solutions of a range from faintly acid to nearly pure lime-water, is oxy- or hydroxy-apatite, but, although its composition may be indicated by the formula 3Ca₃(PO₄)₂.Ca(OH)₂, it is not to be regarded as a basic salt, but rather as the salt of a polymeride of phosphoric acid, H₁₁P₃O₁₃ (or 3H₃PO₄.H₂O), with one hydrogen atom un-neutralised (Compare the relation of orthophosphoric acid, H₆P₂O₈ or H₄P₂O₇.H₂O, to pyrophos- phoric acid, H₄P₂O₇). The slowness of formation of hydroxy-apatite favours the view that it is produced by polymerisation.

Since hydroxy-apatite is the stable phase at the degree of acidity characteristic of animal tissues, it is reasonable to suppose that the phosphate in bones takes this form.1 This applies also to phosphates in soils where the conditions are such that all phosphatic fertilisers should be ultimately transformed into hydroxy-apatite. Earthy phosphorites and coprolites are no doubt impure mixtures of calcium carbonate with hydroxy-apatite, in which some of the hydroxyl has been replaced by chlorine or fluorine. This explains why, after fine grinding, these minerals may be used as fertilisers without further treatment.