THERMODYNAMIC PROPERTIES OF STRONTIANITE-WITHERITE SOLID-SOLUTION (SR,BA)CO3

Structural parameters and thermodynamic properties of strontianite - w itherite solid solutions have been studied by X-ray powder diffraction , heat flux Calvet calorimetry and cation-exchange equilibria techniqu e. X-ray study of the synthetic samples have shown linear and quadrati c (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters d emonstrate the non-ideal character of strontianite - witherite solid s olutions. Enthalpies of solution of the samples have been measured in 2PbOB2O3 at 973 K. The new data on the enthalpy of formation Delta H- f.298.15(0) of SrCO3 and BaCO3 were obtained: - 1231.4 +/- 3.2 and - 1 209.9 +/- 5.8 kJmol(-1) respectively. The enthalpy of mixing of the s olid solution was found to be positive and asymmetric with maximum at X(Ba) (carbonate) = 0.35. The composition dependence of the enthalpy o f mixing may be described by two - parametric Margules model equation: [GRAPHICS] Cation-exchange reactions between carbonates and aqueous S rCl2-BaCl2 supercritical solutions (fluids) were carried out at 973 an d 1073 K and 2 kbar. Calculated Margules model parameters of the exces s free energy are: for orthorhombic carbonate solid solutions W-Sr = W -Ba = 11.51 +/- 0.40 kJmol(-1) (973 K) and W-Sr = W-Ba = 12.09 +/- 0. 95 kJmol(-1) (1073 K); for trigonal carbonate solid solutions W-Sr = W-Ba = 13.55 +/- 0.40 kJmol(-1) (1073 K).