Synthesis and physicochemical study of type A carbonated hydroxyapatites

The CaCl2-(NH4)(2)HPO4-(NH4)(2)CO3-NH3-H2O system at 25 degreesC was studie d by the solubility (residual concentration) method and pH measurements. Th e compounds isolated from the system were characterized by chemical analysi s, X-ray powder diffraction, IR spectroscopy, and NMR spectroscopy. Type A carbonated hydroxyapatites, Ca-10(PO4)(6)(CO3)(x)(OH)(2-2x) (x less than or equal to 1), were identified. As the CO32- ion concentration in the origin al solution is raised, x increases to reach unity in Ca-10(PO4)(6)CO3. The carbonated hydroxyapatites result from a partial or complete replacement of the OH- ions by CO32- ions in the calcium channels of the hydroxyapatite s tructure. No substitution of CO32- ions for PO43- ions was observed.