Thermochemistry of double carbonates in the K2CO3-CaCO3 system

Enthalpies of formation, ΔH, of the double carbonates K2Ca(CO3)2 (with two polymorphs, butschliite and fairchildite) and K2Ca2(CO3)3 from the components K2CO3 and CaCO3 were determined at 298 K by drop solution calorimetry in molten 2PbO·B2O3 at 974 K. The enthalpies of formation were -38.7±3.2 kJ/mol for butschliite. -5.1±3.3 kJ/mol for fairchildite and -7.2±5.8 kJ/mol for K2Ca2(CO3)3. The entropy changes for the formation. ΔSf, were also evaluated for the double carbonates. Assuming ΔSf(butschliite) = 0, because of its ordered structure, the entropy of formation of fairchildite is 40.9±5.1 J/mol.K and of K2Ca2(CO3)3 is 39.4±7.3 J/mol.K.