A NEW METHOD FOR DETERMINING GIBBS ENERGIES OF FORMATION OF METAL-CARBONATE SOLID-SOLUTIONS .1. THE CAXCD1-XCO3(S) SYSTEM AT 298-K AND 1-BAR

New thermodynamic data are presented for mixing of calcite (CaCO3(s)) and otavite (CdCO3(s)) to form Ca(x)Cd1-xCO3(s) solutions. A reversible equilibrium was achieved between the aqueous solution and the metastable solid solution using electrochemical double cells and a new electrode of the third kind. Evidence that the cell reactions are reversible includes demonstration of Nernstian response to changes in electrolyte composition and an absence of significant hysteresis in cell current-voltage and current-temperature (10-35-degrees-C) plots. We observe negative excess Gibbs energies for mixing on the order of 0 to -6.6 kJ.mol-1; these magnitudes are sensitive to uncertainties in the estimates of DELTAG(f)o[CdCO3(s)]. The double-cell method is potentially useful in other systems where, through incorporation of impurities, the activity of the CaCO3(s) is not unity relative to the standard state of the pure, well-crystallized, endmember mineral. It remains to be seen whether the double cell method is equally useful for systems, such as the Ca(x)Mg1-xCO3(s) or Ca(x)Sr1-xCO3(s) solid solutions, that have variable and positive excess Gibbs energies, but some of the same advantages would apply.