INHIBITING EFFECT OF ZINC ON HYDROXYLAPATITE CRYSTALLIZATION

X-Ray diffraction and spectrophotometric analysis have been used to investigate the role of zinc on hydroxylapatite (HA) crystallization. The presence of zinc in solution strongly inhibits the crystallization of hydroxylapatite, which can be synthesized as a unique crystalline phase only up to zinc concentration of about 25 atom %. This phase exhibits a reduction of Ca/P molar ratio and crystal sizes with increasing zinc concentration. Although the Ca/Zn ratio in the solid phase is almost equivalent to that in solution, the values of the cell parameters of the apatitic phase indicate that zinc cannot appreciably substitute for calcium in HA structure. Therefore, zinc must be assumed to be adsorbed on the surface of apatite crystallites and/or in the amorphous phase. The extent of thermal conversion of HA into beta-tricalcium phosphate (beta-TCP) increases with increasing zinc concentration in the solid phase, either when it is obtained by means of synthesis in solution or after cyclic pH fluctuation. The decrease of the lattice constants of beta-tricalcium phosphate with increasing zinc concentration in the solid phase indicates that zinc partially replaces calcium in this structure. The inhibiting effect of zinc on HA crystallization and its preference for beta-TCP structure closely resembles the behavior previously observed for magnesium.