GIBBS ENERGIES OF FORMATION OF METAL-CARBONATE SOLID SOLUTIONS: PART 3. THE CAXMN1-XCO3 SYSTEM AT 298 K AND 1 BAR - THE CAXCD1-XCO3(S) SYSTEM AT 298 K AND 1 BAR

Abstract

New thermodynamic data are presented for the mixing of calcite (CaCO3(s)) and rhodochrosite (MnCO3(s)) to form CaxMn1-xCO3(s) solutions. A reversible equilibrium was achieved between the aqueous solution and the metastable solid solution using electrochemical cells and a new electrode of the third kind.New values of Δf G° [PbCO3(s)] and Δf G° [MnCO3(s)] were determined as -622.61 +/- 0.79 kJ/mol and -809.89 +/- 0.73 kJ/mol, respectively, using the more thoroughly studied value for the Gibbs energy of formation of calcite as a reference. The CaxMn1-xCO3 system was evaluated using these new Gibbs energy values and was found to have thermodynamically stable compositions for 0.186 < x < 0.734, which is consistent with the compositions of natural minerals. X-ray analysis of the solid solutions, before and after the reaction, shows very little evidence of unmixing for time periods in excess of 45 days. The excess Gibbs energy function derived for this system at 25°C is: ΔG°ex=[x(1-x)][(27.84+/-5.12)-(132.62+/-23.47)x+(142.65+/-23.68)x2] where x represents the mole fraction of calcium in the solid solution.The ΔG°ex values found in this study show a strong resemblance to high temperature ΔH°ex values reported by Capabianco and Navrotsky (1987). This similarity suggests that the CaxMn1-xCO3 system has very small ΔS°ex values. However, this conclusion cannot be easily reconciled with the phase equilibrium studies at high temperatures. These phase equilibrium studies indicate a large miscibility gap in intermediate calcium concentrations at high temperatures, in the region where we find evidence for miscibility at 25°C.