Thermochemistry of mixing strontianite [SrCO3(s)] and aragonite [CaCO3(s)] to form CaxSr1-xCO3(s) solid solutions

Enthalpies of mixing [Delta H-298(mix)] of aragonite and strontianite to form CaxSr1-xCO3(s) solid solutions were obtained at 298 K from drop-solution enthalpies [Delta H-975(ds)] of pellets of the respective solid solutions into molten 2PbO . B2O3 at 975 K. The measured Delta H-298(mix) 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 Delta G(298)(ex) 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 approximate to 0.7. The general similarity between the Delta H-298(mix) and the Delta G(298)(ex) 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.