Four synthetic calcium phosphate samples with different Ca/P or cation/P ratios were submitted to 45Ca2+ and 32PO43- exchange for seven days in a standard solution approximating the 'physiologic' concentrations for Ca PO4 Na and Cl. The samples exhibited different affinities for Ca2+ and PO43- directly dependent on the composition of the solid. Optical and x-ray diffraction techniques were employed to detect changes in the solid phases. Variations directly related to the partition of Ca2+ and PO43- into the solids and kinetics of isotope exchange were observed. Relative changes (before and after chemical treatment) were measured after pyrolysis of the samples. Interpretation of the data required knowledge of the phase equilibria in the CaO-P2O5-H2O and CaO-P2O5 systems and the crystal chemistry of calcium phosphate apatites. Bone mineral is subject to long term exchange and equilibration with the surrounding serum. Maturation of bone crystallites may parallel the exchange reactions of the synthetic calcium phosphates.
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