Precipitation and mixing properties of the 'disordered' (Mn,Ca)CO_3solid solution

摘要

The enthalpy of mixing of the calcite–rhodochrosite (Ca,Mn)CO3 solid solution was determined at 25 °C from calorimetricmeasurements of the enthalpy of precipitation of solids with different compositions. A detailed study of the broadening ofpowder X-ray diffraction peaks shows that most of the precipitates are compositionally homogeneous. All the experimentalenthalpy 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 (kJ mol-1 ) where x represents the mole fraction of MnCO3. In addition, the excess volumes of mixing (ΔVex), determined from an X-raydiffraction study of the precipitates, are also positive. Both mixing parameters, ΔHm and ΔVex point towards the existence ofa miscibility gap in the solid solution at ambient temperature and pressure conditions. The positive enthalpies of mixing mea-sured for the precipitates of intermediate compositions dispel the presence of any significant ordering that could be attributedto the precipitation of kutnahorite CaMn(CO3)2. Likewise, the X-ray diffractograms do not show either clear or incipientsuperstructure reflections typical of the kutnahorite ordering. Based on these data, we propose a thermodynamic modelfor this "disordered" (Mn,Ca)CO3 solid solution in which the values of the free energy of mixing (ΔGm) were estimated assum-ing 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 predictedmiscibility gap, a complete series of the disordered solid solution was obtained but most of the precipitates have metastablecompositions. Since the ordered (or partially ordered) phases of intermediate composition described in the literature shownegative enthalpies of formation with respect to a mechanical mixture of the pure end-members, the present ΔHm data sup-port an energetics model that assumes positive interactions (tendency to exsolution) between Ca and Mn within cation layersin the crystal structure, and negative Ca–Mn interactions (tendency to order) between successive cation layers. Finally, thederived mixing properties were used to calculate the aqueous solubility of this solid solution, the results being in good agree-ment with experimental measurements reported in the literature.