Octacalcium phosphate and apatite overgrowths: A crystallographic and kinetic study

Octacalcium phosphate (OCP) and apatite overgrowths were studied using scanning and transmission electron microscopy (SEM and TEM). Precipitates containing overgrowths were prepared using constant composition methods with either OCP or dental enamel apatite seeds. In some cases the OCP-apatite interface was imaged using high-resolution TEM at an approximate 2.5-A resolution. Chemical analysis showed that OCP could be grown on apatite seeds under constant composition conditions, but that apatite growth on OCP although not kept at constant composition, indicated the growth of a phase more basic than OCP with a Ca P molar ratio of 1.50 ± 0.04. High-resolution TEM confirmed that this phase was “apatite-like.” The data show that it is possible to grow apatite on OCP and vice versa near physiological pH. Overgrowths of apatite and OCP are of biological interest as both phases appear to precipitate during hard tissue mineralization.