Abstract. The thermodynamic properties of the copper carbonates malachite and azurite have been studied by adiabatic calorimetry, by heat-flux Calvet Calorimetry, by differential thermal analysis (DTA) and by thermogravimetric (TGA) analysis. The heat capacities, C o of natural malachite and azurite have been measured between 3.8 and 300 K by low-temperature adiabatic calorimetry. The heat capacity of azurite exhibits anomalous behavior at low temperatures. At 298.15 K the molar 8298.15 heat capacities C ~ and the third law entropies o are 228.5 _+ 1.4 and 254.4 _+ 3.8 J mol-1 K-1 for azurite and 154.3+0.93 and 166.3• K -1 for malachite. Enthalpies of solution at 973 K in lead borate (2PbO'BzO3) have been measured for heat treated malachite and azurite. The enthalpies of decomposition are 105.1 +__ 5.8 for azurite and 66.1 +_ 5.0 kJ tool-1 for malachite. The enthalpies of formation from oxides of azurite and malachite determined by oxide melt solution calorimetry, are -84.7 + 7.4 and -52.5 __+ 5.9 kJ mol-1, respectively. On the basis of the thermodynamic data obtained, phase relations of azurite and malachite in the system Cu 2 + -HzO-CO2 at 25 and 75 ~ have been studied.

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