Gibbs energies of formation of metal-carbonate solid solutions 2: The CAXSR1-ZCO3(S) system at 298 K and 1 bar

Abstract

New thermodynamic data are presented for the Gibbs energies of formation (ΔGof[CaxSr1-xCO 3(s)]) and the excess Gibbs energies of formation (GE) for CaxSr1-xCO3(s) solutions. Reversible equilibria were achieved between CaCl2(aq) + SrCl2(aq) solutions and carbonate electrodes of the third kind in electrochemical cells of the type: Pb(Hg, 2 - phase)|PbCO3(s), CaxSr1-xCO3(s)|SrCl2(m1), CaCl2(m2), |Hg2Cl2(s)|Hg(1) In most cases, we found no evidence in X-ray diffractograms for incongruent dissolution of the solids to yield strontian aragonites or calcite over the period of the cell measurements. Although secondary calcite forms in some experiments, the unmixing is sufficiently slow (weeks of reaction) that meaningful ΔGof[CaxSr1-xCO 3(s)] values can be determined. We observe positive GE values over the range 0 < x < 0.9 such that intermediate compositions in the SrCO3-CaCO3 binary are unstable relative to a mechanical mixture consistent with the compositions of natural minerals. The GE values determined in this study agree with those determined by Plummer and Busenberg (1987) to within the respective experimental uncertainties. The GE values that we report remain unchanged, if the values of ΔGof[SrCO3(s)] and ΔGof[Sr2+(aq)] are revised in a compensating manner in a later study.