Magnesium-containing carbonate apatites.

Hydroxyapatites precipitated at pH 7.0 and 9.0 with and without carbonate and with different amounts of magnesium were studied. Mg uptake, Ca/P ratios, and lattice constant data indicate that Mg is incorporated into the apatite lattice. IR spectra demonstrate the formation of B-type carbonate apatites with carbonate substituting for phosphate. Decomposition of carbonate-containing apatites at elevated temperatures up to 1000 degrees C is more gradual for apatites prepared at pH 9.0 than for those prepared at pH 7.0 for which an abrupt loss of carbonate occurs after 600 degrees C. Compounds synthesized without added carbonate partially transform to beta Ca3(PO4)2 (TCP) at about 700 degrees C. Greater transformation to TCP occurs as the Mg incorporation is increased, indicating the insertion of Mg into TCP and consequent stabilization of the TCP. SEM micrographs show increases in the size of crystallites when apatites are precipitated with Mg (in the 0.2-1.5% range), providing further evidence for Mg incorporation into the apatite structure.