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

Princeton University, Department of Geosciences and Princeton Minerals Institute, Princeton, NJ, United States

Enthalpies of formation, Delta H, of the double carbonates K ₂ Ca(CO ₃ ) ₂ (with two polymorphs, butschliite and fairchildite) and K ₂ Ca ₂ (CO ₃ ) ₃ from the components K ₂ CO ₃ and CaCO ₃ were determined at 298 K by drop solution calorimetry in molten 2PbO.B ₂ O ₃ at 974 K. The enthalpies of formation were -38.7+ or -3.2 kJ/mol for butschliite. -5.1+ or -3.3 kJ/mol for fairchildite and -7.2+ or -5.8 kJ/mol for K ₂ Ca ₂ (CO ₃ ) ₃ . The entropy changes for the formation. Delta S _f , were also evaluated for the double carbonates. Assuming Delta S _f (butschliite) = 0, because of its ordered structure, the entropy of formation of fairchildite is 40.9+ or -5.1 J/mol.K and of K ₂ Ca ₂ (CO ₃ ) ₃ is 39.4+ or -7.3 J/mol.K.

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