Precipitation and mixing properties of the “disordered” (Mn,Ca)CO3 solid solution

The enthalpy of mixing of the calcite-rhodochrosite (Ca,Mn)CO3 solid solution was determined at 25 °C from calorimetric measurements of the enthalpy of precipitation of solids with different compositions. A detailed study of the broadening of powder X-ray diffraction peaks shows that most of the precipitates are compositionally homogeneous. All the experimental enthalpy of mixing (ΔHm) values are positive and fit reasonably well (R2 = 0.86) to a Guggenheim function of three terms:ΔHm(x)=x(1-x)[3.93+1.97(2x-1)-2.19(2x-1)2]RT(kJmol-1) where x represents the mole fraction of MnCO3. In addition, the excess volumes of mixing (ΔVex), determined from an X-ray diffraction study of the precipitates, are also positive. Both mixing parameters, ΔHm and ΔVex, point towards the existence of a miscibility gap in the solid solution at ambient temperature and pressure conditions. The positive enthalpies of mixing measured for the precipitates of intermediate compositions dispel the presence of any significant ordering that could be attributed to the precipitation of kutnahorite CaMn(CO3)2. Likewise, the X-ray diffractograms do not show either clear or incipient superstructure reflections typical of the kutnahorite ordering. Based on these data, we propose a thermodynamic model for this “disordered” (Mn,Ca)CO3 solid solution in which the values of the free energy of mixing (ΔGm) were estimated assuming random mixing. From the ΔGm(x) function, the miscibility gap was calculated to range from x = 0.13 to 0.98. This gap is “metastable” as it covers a compositional range within which ordered kutnahorite is the stable phase. Despite the predicted miscibility gap, a complete series of the disordered solid solution was obtained but most of the precipitates have metastable compositions. Since the ordered (or partially ordered) phases of intermediate composition described in the literature show negative enthalpies of formation with respect to a mechanical mixture of the pure end-members, the present ΔHm data support an energetics model that assumes positive interactions (tendency to exsolution) between Ca and Mn within cation layers in the crystal structure, and negative Ca-Mn interactions (tendency to order) between successive cation layers. Finally, the derived mixing properties were used to calculate the aqueous solubility of this solid solution, the results being in good agreement with experimental measurements reported in the literature.