Thermodynamic Data for Dolomite and Kutnahorite from the Experimental Decarbonation Curves

The P-T stability field of rhombohedral double carbonates, CaMg (CO₃)₂ (dolomite) and CaMn (CO₃)₂ (kutnahorite) has been studied up to 2 kb of PCO₂. The dolomite decomposes to CaCO₃ (ss)+MgO+CO₂ and the kutnahorite to CaCO₃ (ss)+MnO+CO₂. The kutnahorite was considered to be completely disordered (R̅₃C) near the decomposition temperature while the dolomite was found to be in ordered state at the decomposition temperature, though both were considered to stabilise with ordered structures at lower temperatures. The standard enthalpy and free energy data retrieved for the ordered dolomite is ; ΔH⁰_f = -2331.64±2.07 kJ/mol, ΔG⁰_f = -2168.59±2.07 kJ/mol., and for the disordered kutnaborite, is; ΔH⁰_f = -2093.46±3.77kJ/mol, ΔG⁰_f = -1944.80±3.77kJ/mol. The new data on dolomite are atIeast 7 kJ more negative than the earlier reported values. In the low temperature region the double carbonate field is preceded by a metastable two carbonate field of M'CO₃ (ss) and M"CO₃ (M' = Ca, M" = Mg, Mn) phases. This is considered to be the likely stage in the natural process of dolomite or kutnahorite formation.