Influence of foreign metal ions on crystal growth and morphology of brushite (CaHPO4, 2H2O) and its transformation to octacalcium phosphate and apatite

Brushite, forming tabular crystals, has been precipitated at 25 °C in the presence of each of 14 different di- and trivalent metal ions. The influence of these ions at micromolar concentrations on the solvent-mediated phase transformation of brushite to more basic calcium phosphates has been studied as well. The effect of additives on brushite crystallization was pH-dependent, which could be related to the presence or absence of amorphous precipitate. In the latter case the course of the crystallization process could be followed by recording pH as function of time. For half of the additives kinetic analysis was possible and showed that the crystal growth mechanism is surface nucleation. Edge free energy is lowered in the presence of an additive. Zn favoured aggregates, and the transition metals with the exceptions of Mn(II), Co(II) and Cu(II) favoured irregular growth. Zn inhibited lateral growth, as did Cd and Cr(III) at low and Cu(II) at high pH. Most of the ions have a marked effect on the transformation to octacalcium phosphate (OCP) and hydroxyapatite (HAP) as well. In both cases Cu(II) and Zn are strong inhibitors, whereas Pb(II) is a moderate promotor. Fe in both oxidation states, Co(II), Mn(II) and Sr are intermediate in effect on phase transformation. Inhibition may be caused by adsorbed foreign ions impeding growth of nuclei or by poisoning of the substrate for heterogeneous nucleation, i.e. brushite crystals. Promotion is explained by the formation of nuclei with suitable crystal structure, e.g. apatite/pyromorphite (Ca,Pb)5OH(PO4)3 in the case of Pb.