Thermochemistry of mixing strontianite [SrCO₃(S)] and aragonite [CaCO₃(S)] to form Ca_xSr_1-xCO₃(S) solid solutions

Enthalpies of mixing [ΔH^mix₂₉₈] of aragonite and strontianite to form Ca_xSr_1-xCO₃(S) solid solutions were obtained at 298 K from drop-solution enthalpies [ΔH^ds₉₇₅] of pellets of the respective solid solutions into molten 2PbO·B₂O₃ at 975 K. The measured ΔH^mix₂₉₈ 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 ΔG^ex₂₉₈ 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 ΔH^mix₂₉₈ and the ΔG^ex₂₉₈ 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 Ca_xSr_1-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.