A-type carbonate hydroxyapatites with 0 ⩽ x ⩽ 1 were prepared and characterized by DRX, IR spectroscopy and CHN analysis. The heat of solution was measured in 9 wt% HNO3 using an isoperibol calorimeter. The standard enthalpy of formation was determined by thermochemical cycle. Gibbs free energy has been deduced by estimating standard entropy of formation. Carbonatation increases the stability till x = 0.6 mol.