Solid-solution thermodynamics in CaCO₃-MnCO₃.

Thermodynamic mixing data are reported for the calcite-rhodochrosite solid-solution series. Enthalpies of solution in an aqueous solvent saturated in NaCl and containing 0.745 M HCl were obtained for CaCO₃-MnCO₃ samples prepared at 600oC. Positive excess free energies and complex enthalpies (both endo- and exothermic deviations) imply negative excess entropies of mixing. Asymmetric enthalpy behaviour is inferred from phase-equilibrium data. This behaviour is seen in materials without the long-range Mn-Ca ordering characteristic of kutnahorite.-J.A.Z.