INHIBITING EFFECT OF ZINC ON HYDROXYLAPATITE CRYSTALLIZATION

X-Ray diffraction and spectrophotometric analysis have been used to in vestigate the role of zinc on hydroxylapatite (HA) crystallization. Th e presence of zinc in solution strongly inhibits the crystallization o f hydroxylapatite, which can be synthesized as a unique crystalline ph ase only up to zinc concentration of about 25 atom %. This phase exhib its a reduction of Ca/P molar ratio and crystal sizes with increasing zinc concentration. Although the Ca/Zn ratio in the solid phase is alm ost 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 ad sorbed on the surface of apatite crystallites and/or in the amorphous phase. The extent of thermal conversion of HA into beta-tricalcium pho sphate (beta-TCP) increases with increasing zinc concentration in the solid phase, either when it is obtained by means of synthesis in solut ion or after cyclic pH fluctuation. The decrease of the lattice consta nts of beta-tricalcium phosphate with increasing zinc concentration in the solid phase indicates that zinc partially replaces calcium in thi s structure. The inhibiting effect of zinc on HA crystallization and i ts preference for beta-TCP structure closely resembles the behavior pr eviously observed for magnesium.