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 reversibl e equilibrium was achieved between the aqueous solution and the metast able solid solution using electrochemical double cells and a new elect rode of the third kind. Evidence that the cell reactions are reversibl e includes demonstration of Nernstian response to changes in electroly te composition and an absence of significant hysteresis in cell curren t-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 estimat es of DELTAG(f)o[CdCO3(s)]. The double-cell method is potentially usef ul in other systems where, through incorporation of impurities, the ac tivity 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 va riable and positive excess Gibbs energies, but some of the same advant ages would apply.