Formation of carbonated apatite in equilibrium system CaO–P2O5–H2CO3–H2O at 298 K in air

Equilibrium in the quaternary CaO–P2O5–H2CO3–H2O system at 298 K in air was studied in a region of low concentrations of liquid-phase components. We revealed a dependence of the composition of resultant equilibrium solid phases on the Ca/P ratio of the initial components and on pH of the equilibrium liquid phase. It was found that the entire region of the system having low concentrations of liquid-phase components is occupied by variable-composition compounds with apatite structure. Two types of carbonate-containing apatite compounds were detected. The first type of apatite forms at 5.4 ≤ pH ≤ 7.0, contains HPO42− groups in the lattice, and has an invariant point with brushite CaHPO_4·2H_2O at pH ≈ 5.4. The second type arises at pH > 7.0, does not contain an acidic group, and has an invariant point with CaCO3 at pH ≥ 8.