Characterization and crystallization of Ba(SO4,SeO4) solid solution

Investigation of the equilibrium thermodynamics of the Ba(SO4, SeO4)-H2O system has shown that sulfate ions partition preferentially into the solid solution. When crystallized under supersaturated conditions, the solid phase is, however, enriched in BaSeO4 With respect to equilibrium compositions. The crystals exhibit sectorial zoning superimposed on concentric zoning. The solid solution was studied using precipitation experiments and crystallization in gel. Powder XRD analysis of the crystals formed in the precipitation experiments show that the unit cell parameters vary almost linearly with composition and that the excess volume of mixing is almost zero, suggesting that this solid solution is almost ideal. The crystals grown at high supersaturations in the gel are enriched in BaSeO4 showing that under these conditions a deviation from equilibrium partitioning occurs. The crystals also exhibit concentric zoning with a BaSO4-rich core surrounded by BaSeO4-rich regions and, in crystals grown from solutions high in sulfate, a BaSO4-rich rim. Sectorial zoning is superimposed on the concentric zoning and is thought to be caused by preferential incorporation of BaSO4 onto the {210} form which is important in the BaSO4 endmember. The morphological changes of this solid solution as a function of composition are also described.