Thermochemistry of double carbonates in the K 2 CO 3 -CaCO 3 system

Enthalpies of formation, DHf, of the double carbonates K2Ca(CO3)2 (with two poly- morphs, butschliite and fairchildite) and K 2Ca2(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 238.7 6 3.2 kJ/mol for butschliite, 25.1 6 3.3 kJ/mol for fairchildite and 27.2 6 5.8 kJ/mol for K2Ca2(CO3)3. The entropy changes for the formation, DSf, were also evaluated for the double carbonates. Assuming DSf(butschliite)5 0, because of its ordered structure, the entropy of formation of fairchild- ite is 40.9 6 5.1 J/mol·K and of K2Ca2(CO3)3 is 39.4 6 7.3 J/mol·K.