GIBBS ENERGIES OF FORMATION OF METAL-CARBONATE SOLID-SOLUTIONS - PART3 - THE CAXMN1-XCO3 SYSTEM AT 298 K AND 1 BAR

New thermodynamic data are presented for the mixing of calcite (CaCO3( s)) and rhodochrosite (MnCO3(s)) to form CaxMn1-xCO3(s) solutions. A r eversible equilibrium was achieved between the aqueous solution and th e metastable solid solution using electrochemical cells and a new elec trode of the third kind. New values of Delta(f) G degrees [PbCO3(s)] a nd Delta(f) G degrees [MnCO3(s)] were determined as -622.61 +/- 0.79 k J/mol and -809.89 +/- 0.73 kJ/mol, respectively, using the more thorou ghly studied value for the Gibbs energy of formation of calcite as a r eference. The CaxMn1-xCO3 system was evaluated using these new Gibbs e nergy values and was found to have thermodynamically stable compositio ns for 0.186 < x < 0.734, which is consistent with the compositions of natural minerals. X-ray analysis of the solid solutions, before and a fter the reaction, shows very little evidence of unmixing for time per iods in excess of 45 days. The excess Gibbs energy function derived fo r this system at 25 degrees C is: Delta G degrees(ex) = [x(1 - x)][(27 .84 +/- 5.12) - (132.82 +/- 23.47)x + (142.65 +/-23.68)x(2)] where x r epresents the mole fraction of calcium in the solid solution. The Delt a G degrees(ex) values found in this study show a strong resemblance t o high temperature Delta H degrees(ex) values reported by Capabianco a nd Navrotsky (1987). This similarity suggests that the CaxMn1-xCO3 sys tem has very small Delta S degrees(ex) values. However, this conclusio n cannot be easily reconciled with the phase equilibrium studies at hi gh temperatures. These phase equilibrium studies indicate a large misc ibility gap in intermediate calcium concentrations at high temperature s, in the region where we find evidence for miscibility at 25 degrees C. Copyright (C) 1998 Elsevier Science Ltd.