Experimental data of Latil and Maury (1977), Korzhinskiy (1981), and Ruszala and Kostiner (1975) on the hydroxyapatite-chlorapatite (HAp-ClAp) solid solution were modeled as symmetric and asymmetric solutions. Results of Latil and Maury (1977) and Kor-zhinskiv (1981) on the hydroxyapatite-fluorapatite (HAp-FAp) join were also modeled.

The regular solution model fits the HAp-ClAp data without obvious complications, yielding binary interaction parameters (WG) of less than 20 kJ·mol−1·K−1. The data suggest that the HAp-ClAp solid solution may be treated as essentially ideal at or above 500 °C. Using a heat-capacity function calculated from that of FAp, the data may be reduced to give an enthalpy of formation for ClAp of –6548.027 kJ°mol−1·K−1 and an entropy of 457 J·mol−1 at 298.15 K and 1 bar.

Data on the HAp-FAp join are not conclusive, but allow the interpretation of ideal solution. Consideration of the details of F-OH interaction support this interpretation. The presence of Na as a component in the experiments of Latil and Maury (1977) is an insurmountable obstacle to modeling their results. The reversal experiments of Korzhinskiy (1981) show that FAp essentially did not re-equilibrate at 500 °C and 600 °C, in agreement with the results of earlier workers. The failure of Korzhinskiy’s experiments to reach equilibrium at 700 °C is a useful indication of FAp behavior under hydrothermal conditions.

For all practical purposes, ClAp-HAp and HAp-FAp solid Solutions may be treated as ideal at geologie temperatures and pressures.

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