Heterogeneous synthesis of fluoridated hydroxyapatites.

Fluoridated hydroxyapatites were synthesized with two different modes of fluoride supply: by supplying F(-)-free solution initially, followed by a F(-)-containing solution whose fluoride concentration was stoichiometrically equal to that of fluorapatite; and with the order of supply of these solutions reversed. Both of these heterogeneously synthesized fluoridated hydroxyapatites showed typical calcium phosphate X-ray diffraction patterns; and both had similar total fluoride contents (0.94 +/- 0.02 and 0.99 +/- 0.03 mmol/g, respectively), i.e. half of the maximum fluoride content of fluorapatite. However, they differed considerably in their physico-chemical properties. The former had a wider peak breadth of X-ray diffraction and a lower apparent solubility (Ca = 12.2 +/- 0.7 mmol/l) than the latter (Ca = 19.8 +/- 0.4 mmol/l) at 37 degrees C and pH 4.0. Wavelength dispersive spectroscopy attached to scanning electron microscopy gave clearly different spectra. Electron spectroscopy for chemical analysis of apatite pellets made by pressing and heating showed a slightly larger F1s signal at the crystal surface of the former, although the difference was not marked. These results suggest that two different types of heterogeneous fluoridated hydroxyapatites were formed, hydroxyapatite covered with fluorapatite and fluorapatite covered with hydroxyapatite.