Structural parameters and thermodynamic properties of strontianite -witherite solid solutions have been studied by X-ray powder diffraction, heat flux Calvet calorimetry and cation-exchange equilibria technique. X-ray study of the synthetic samples have shown linear and quadratic (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters demonstrate the non-ideal character of strontianite -witherite solid solutions. Enthalpies of solution of the samples have been measured in 2PbO, B203 at 973 K. The new data on the enthalpy of formation AH~ of SrCO 3 and BaCO 3 were obtained: -1~ 1.2~___ 3.2 and -1209.9 β’ 5.8 kJ, mol-1 respectively. The enthalpy of mixing of the solid solution was found to be positive and asymmetric with maximum at XBa (carbonate)=0.35. The composition dependence of the enthalpy of mixing may be described by two -parametric Margules model equation:
AHmi
x = XBa * Xsr 9 [(4.40 + 3.91) 9 XBa + (28.13 _+ 3.91) * XSr ] kJ * mol-1 Cation-exchange reactions between carbonates and aqueous SrC12-BaC12 supercritical solutions (fluids) were carried out at 973 and 1073 K and 2 kbar. Calculated Margules model parameters of the excess free energy are: for orthorhombic carbonate solid solutions Wsr=WBa=ll.51+0.40kJ*mo1-1 (973 K) and Wsr= W~,= 12.09_+0.95 kJ,mo1-1 (1073 K); for trigonal carbonate solid solutions WSr = WBa = 13.55___ 0.40 kJ* mol-1 (1073 K).