NUCLEATION, GROWTH, AND ZONING PHENOMENA IN CRYSTALLIZING (BA,SR)CO3, BA(SO4,CRO4), (BA,SR)SO4, AND (CD,CA)CO3 SOLID SOLUTIONS FROM AQUEOUS SOLUTIONS

Abstract

Crystals of (Ba,Sr)CO3, Ba(SO4,CrO4), (Ba,Sr)SO4, and (Cd,Ca)CO3 solid solutions were obtained by counterdiffusion of reactants through a column of porous silica hydrogel. For each system, a set of experiments, starting with mother solutions of different concentrations, was carried out. The composition of the solids was analyzed by electron microprobe. The method allows the establishment of the influence of the supersaturation on the distribution of material between solid and aqueous phases. With this aim, equilibrium calculations and effective composition of the nucleating crystals are compared.Two extreme nucleation behaviours have been observed. The solid solutions (Ba,Sr)SO4 and (Cd,Ca)CO3, with endmember solubility products differing by several orders of magnitude, tend to nucleate from aqueous solutions in a bimodal way. In these systems there is a strong preferential partitioning and only a very narrow range of aqueous-phase compositions can coexist in equilibrium with intermediate solid solutions. At high supersaturations, the range of aqueous solutions from which intermediate solid solutions can nucleate expands, but the bimodal effect remains. At the other extreme, the solid solutions with close endmember solubility products, like (Ba,Sr)CO3 and Ba(SO4,CrO4), tend to nucleate in an evenly distributed way. For these solid solutions, a wide range of fluid compositions is in equilibrium with intermediate solid solutions. When nucleation occurs at high supersaturation, the substituting ions incorporate into the nuclei nearly in the same stoichiometric proportion as in the aqueous phase, and the partition coefficients reach values approaching unity.Finally, the growth process has been studied by monitoring the compositional evolution of the crystals. There is a close relationship between the bimodal trend of a solid solution and the appearance of high gradient compositional zoning. The solid solutions (Ba,Sr)SO4 and (Cd,Ca)CO3 develop sharp compositional zoning when the fluid composition passes through a certain range during the growth process. On the contrary, when the solid solutions (Sr,Ba)CO3 and Ba(SO4,CrO4) grow at high supersaturations, the solids have nearly the same stoichiometry as the aqueous phase, and the crystals tend to be homogeneous or, at most, to undergo a very gradual compositional evolution.