Enthalpies of mixing [ΔHmix298] of aragonite and strontianite to form CaxSr1-xCO3(S) solid solutions were obtained at 298 K from drop-solution enthalpies [ΔHds975] of pellets of the respective solid solutions into molten 2PbO·B2O3 at 975 K. The measured ΔHmix298 values are positive for all measured values of x, are nearly symmetric around x - 0.50, and reach a maximum value of +3.82 ± 0.94 kJ mol-1. Previous electrochemical studies have reported that ΔGex298 values are also positive over the range 0.0 < x < 0.9 and reach a maximum value of +3.0(±1.6) kJ mol-1 at x ≈ 0.7. The general similarity between the ΔHmix298 and the ΔGex298 values indicates that the excess entropy of mixing is small or zero, consistent with the regular-solution treatment. Within this regular-solution treatment, the interaction parameter is W= 13.5(±1.3) kJ mol-1, which yields a very narrow range of stable miscible compositions at Earth surface conditions. Compositions of aragonite or strontianite with even a few percent impurity are not stable and will unmix to form a mechanical mixture of Ca-rich strontianite and a Sr-rich aragonite. It is, in general, difficult to accurately estimate the compositions of coexisting, miscible solids such as the CaxSr1-xCO3 solids at Earth-surface conditions. The predictions are quite sensitive to small uncertainties in the data and such estimates are probably beyond the capabilities of either electrochemical cell measurements or calorimetry.
ASJC Scopus subject areas
- Geochemistry and Petrology