Crystallisation of Ba(SO 4, CrO 4) solid solutions from aqueous solutions

Abstract

Barite (BaSO 4) and barium chromate form a complete solid solution series, where SO 4 2− ions substitute for CrO 4 2−. Both endmembers are isomorphous and crystallise in the space group Pnma. In this work we present an experimental study of the crystallisation behaviour of this solid solution from aqueous solutions. Crystals of Ba(SO 4,CrO 4) were grown by counter-diffusion of BaCl 2 and Na 2SO 4+Na 2CrO 4 through a column of silica hydrogel. A set of experiments starting with parent solutions of different SO 4 2−/CrO 4 2− ratios was carried out. The composition of the obtained crystals was studied by electron microprobe. A linear influence of the composition on the structural parameters was determined by X-ray diffraction. The equilibrium partitioning of sulphate between the aqueous and the solid phase can be described by means of a function X BaSO 4 ( X SO 4,aq ), which relates the solid mole fraction and the aqueous activity fraction of sulphate. According to this function there is a moderate preferential partitioning of sulphate towards the solid phase. However, at high supersaturations SO 4 2− and CrO 4 2− ions tend to lie down in a ratio which approximates to the unity. As a consequence, the substituting ions incorporate into the solid in a stoichiometric proportion approaching that of the aqueous phase, and the obtained crystals are relatively enriched in the more soluble endmember BaCrO 4.