CRYSTALLIZATION OF CALCIUM AND MAGNESIUM PHOSPHATES FROM SOLUTIONS OF LOW CONCENTRATION

The precipitation and evolution of calcium phosphates in solutions of low concentration ([Ca] = [P] = 0.005M) were studied at 25° C in the presence of magnesium the concentration of which ranged from 0.5mM to 50mM. The initial and final phases are described in terms of initial composition and pH of solutions. The first precipitating phase is an amorphous calcium phosphate, Ca3(PO4)2·nH2O, which transforms into apatite-like phosphate, Ca5(PO4)3OH, or whitlockite, Ca9MgH(PO4)7, or remains unaltered, according to the magnesium concentration. Brushite, CaHPO4·2H2O, dominates in the poorly supersaturated solutions. Octacalcium phosphate, Ca4H(PO4)3·2.5H2O, rarely occurs. The conditions for phase stability, and the role of magnesium and supersaturation are discussed. © 1990.