STOICHIOMETRY OF NA+- AND CO32--CONTAINING APATITES OBTAINED BY HYDROLYSIS OF MONETITE

In this study, a series of Na+- and CO32--containing calcium hydroxyapatites were prepared under homogeneous precipitation conditions by the hydrolysis of CaHPO4 in Na2CO3 solutions. A chemical and physical analysis showed that the precipitates were single-phase and well-crystalline B-type carbonated apatites with a carbonate content ranging from 8 to 20 wt %. However, the samples still contained an appreciable amount of water even after prolonged drying under vacuum at 25 degrees C. On the basis of their relative composition and the results of the physical analysis it was found that the predominant substitution mechanisms responsible for the incorporation of Na+ and CO32- in Ca-10(PO4)(6)(OH)2 are Ca2+ + PO43- + OH- <-> V-Ca + CO32- + V-OH and Ca2+ + PO43- <-> Na+ + CO32-, where V-X stands for a vacancy in the X-sublattice. In contrast to the results of other investigations, the ratio, the contributions of these mechanisms to the stoichiometry of the carbonated apatites varies with the total substitution degree of carbonate.