Crystallization behaviour of the (Mn,Ca)CO₃ solid solution in silica gel: nucleation, growth and zoning phenomena

Abstract

Abstract The nucleation and growth behaviour of the (Mn,Ca)CO 3 solid solution from aqueous solutions in silica hydrogel were studied at 25ºC. In order to obtain different levels of supersaturation at nucleation, experiments were carried out in diffusion columns of different lengths. Zoning phenomena, compositional heterogeneity and morphological modifications of the crystals obtained were examined by electron microprobe and scanning electron microscopy and microanalyses. The large difference between the solubility of calcite (CaCO 3 ) and rhodochrosite (MnCO 3 ) implies a significant preferential partitioning of Mn towards the solid. The distribution of Mn 2+ and Ca 2+ ions between the aqueous and solid phase demonstrates that at elevated supersaturation, this preferential partitioning is softened. The morphologies of experimentally grown individuals show a clear dependence on the composition of the parent solution. Crystals obtained from aqueous solutions with large Mn contents exhibit a relatively sharp concentric zoning along the equatorial sections. On these bases, the roles of multiple kinetic and thermodynamic factors during nucleation and growth are also discussed. Finally, an X-ray diffraction analysis of the precipitates provided no evidence of the formation of the dolomite-type ordered kutnahorite [CaMn(CO 3 ) 2 ] in any of the experimentalsets.