Heterogeneous Mg-containing hydroxyapatites.

Two different Mg-containing hydroxyapatites, H-MgAp and Mg-HAp, were synthesized at 80 degrees C and pH 7.4. For H-MgAp, the calcium solution was supplied in the first half of the synthesis period and the calcium solution containing magnesium was supplied in the second half. For Mg-HAp, the order of supply was reversed. The X-ray diffraction patterns of H-MgAp and Mg-HAp differed slightly from each other. The (002) reflection of both patterns shifted towards a higher angle. The chemical compositions, especially the Mg contents, of the two apatites were almost the same as each other; and the Mg contents were half of that of homogeneous MgHAp synthesized with continuous supply of Mg at the same concentration. However, their crystal shapes were quite different, with H-MgAp appearing plate-like and Mg-HAp needlelike. In a solubility experiment, Mg-HAp was more soluble than H-MgAp. These results suggest that two different heterogeneous Mg-containing hydroxyapatites may be formed: hydroxyapatite covered with Mg-containing hydroxyapatite, and Mg-containing hydroxyapatite covered with hydroxyapatite.