Spherulitic Growth of Apatite in a MgO─CaO─SiO2─P2O5 Glass

Differential thermal analysis, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and transmission electron microscopy were used to study the crystallization of a glass with a composition of 11.2 wt% MgO, 40.5 wt% CaO, 33.3 wt% SiO₂, and 15 wt% P₂O₅. A two-phase "composite," which was composed of apatite and an intermediate phase (H-phase), was formed under appropriate heat-treatment conditions. The spherulitic morphology of apatite phase transformed from "open sheaf" into ellipsoidal as samples were heated to a higher temperature. These phenomena were due to the intermediate H-phase becoming unstable at this temperature so that the retardation effect on the apatite dendritic growth disappeared.