Hydroxyapatites with various magnesium contents were synthesized at 80 °C and pH 7.4 to study their physicochemical stability. The contraction of their c-axis dimensions observed in X-ray diffraction analysis, together with the result of chemical analysis, suggested that Mg2+ ions are substituted into the Ca2+ positions. The apatites became less well crystallized and more unstable with the increase of magnesium content. The amorphous sample synthesized at the magnesium-calcium feed molar ratio of 1 revealed a tricalcium phosphate pattern after ignition at 500 °C, while the other samples maintained apatitic features. The apparent solubility at 37 °C in 0.5 M acetate buffer solution of pH 4.0 increased with the increase of magnesium conte
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