Relationships between crystal morphology and composition in the (Ba,Sr)SO4–H2O solid solution–aqueous solution system

Abstract

This paper deals with the relationship between habit and compositional changes in crystals of the BaxSr1xSO4 solid solution at 25 8C and 1 atm. Crystals corresponding to the whole range of compositions of the BaxSr1xSO4 solid solution have been grown in a diffusing–reacting system. The physicochemical evolution resulting from counter-diffusion of SrCl2/BaCl2 and Na2SO4 reactants along a silica hydrogel column results in the development of spatio-temporal crystallization sequences. The external habit and the internal zoning reflecting both the morphological and compositional evolution of the crystals have been studied by scanning electron microscopy, EDAX and microprobe analysis. As composition varies from barite to celestite end-members, a clear evolution both in internal crystal morphologies and in final habit of crystals grown along the gel column has been observed. The main change observed is in the relative development of {001} and {011} forms, which is accompanied by a change in the crystals elongation. While tabular habits dominated by the {001} pinacoid correspond to crystal compositions close to pure barite, crystals with compositions closer to celestite show elongated morphologies dominated by the {011} rhombic prism. This variation is also evident in the internal morphologies of the crystals. The progressive predominance of {011} forms is crystals become richer in Sr has been interpreted on the basis of surface structure description by Hartman–erdok theory. Moreover, Atomic Force Microscopy investigations of the epitaxial growth of pure celestite on barite (001) surfaces under different degrees of supersaturation with respect to celestite have provided complementary quantitative information. Growth morphologies for different supersaturations with respect to celestite can be predicted using growth rates of crystal faces measured on a molecular scale. The results clearly show the effect of Sr incorporation into barite structure on the BaxSr1xSO4 crystal habit at a molecular scale. The results presented in this paper can help to develop morphological criteria relating morphology and composition of BaxSr1xSO4 crystals occurring in natural environments as a result of inorganic crystallization and biomineralization processes.