Stability of Calcium-Strontium Hydroxyapatite Solid Solutions in Aqueous Solutions at 25°

Phase diagrams were constructed from phase equilibrium calculations for the quaternary system Ca(OH)2-Sr (OH)2-H3PO4-H2O and for the system Ca(OH)2-Sr(OH)2-H3PO4-HX(BOH)-H20 at 25°. On the basis of these diagrams the stability field of solid solutions of calcium and strontium hydroxyapatite was derived. The calculations show the borderlines of the apatite field to be determined by the activity isotherms of the sparingly soluble phosphates CaHPO4.2H2O, Ca8H2(PO4)6.5H2O, β-SrHPO4 or Sr6H3(PO4)5.2H2O depending on the pH and the molar cSr:cCa and cCa: cP ratios in the solution. The biological implications of these results are discussed.