Transmission electron microscopic observation of heterogeneous fluoridated hydroxyapatites

Two different types of heterogeneous fluoridated hydroxyapatites previously synthesized at 80 °C and pH 7.4, which might be composed of hydroxyapatite covered with fluorapatite (H-FAp) and fluorapatite covered with hydroxyapatite (F-HAp), were observed by high-resolution transmission electron microscopy. The transverse-sectional shapes of both apatites were clearly hexagonal. This confirmed that both apatites were highly crystallized. However, their shapes differed slightly from each other in detail:H-FAp was more slender and F-HAp was typically hexagonal, probably reflecting the shapes of the original core apatites, hydroxyapatite and fluorapatite. The portions thought to be synthesized with fluoride-free solution showed clear electron beam damage. The expanded lattice image observed at the edge of the F-HAp crystal coincided approximately with the a-axis dimension of 0.9443 nm obtained from the X-ray diffraction data of synthetic hydroxyapatite. After incubation in 0.5 mol l−1 acetate buffer solution (37 °C, pH 4.0) for one month, the fluorapatite-covered H-FAp maintained its sharp corners, while the hydroxyapatite-covered F-HAp showed rounded corners.