Crystallographic behaviour of fluoridated hydroxyapatites containing Mg2+ and CO3(2-) ions

Fluoridated hydroxyapatites containing small amounts of magnesium and carbonate ions were synthesized at 80 and 60 degrees C to examine their inhibiting properties regarding apatite crystal growth, in contrast to the promoting action of fluoride. The shortening of a-axis and c-axis dimensions of the apatite crystals, as revealed by X-ray diffraction analysis, suggested that both magnesium and carbonate ions were substituted into the apatite crystals. The a-axis dimensions also decreased with the degree of fluoridation. The infrared spectra due to CO3(2-) ions at 875 cm-1 were shifted with increasing fluoride content. The overall crystallinity was inhibited in comparison with that of Mg and CO3-free fluoridated hydroxyapatites, but recovered considerably with increased fluoride content. The apparent solubility of the apatites at pH 4.0 and 37 degrees C was higher than that of Mg and CO3-free fluoridated hydroxyapatites at lower fluoride contents, but gradually approached the latter at higher fluoride content. After 1 month's incubation, dicalcium phosphate dihydrate was formed from fluoride-free Mg-CO3 apatite synthesized at 60 degrees C.