Heterogeneous growth of cadmium and cobalt carbonate phases at the calcite surface

The ability of surface precipitates to form heteroepitaxially is an important factor that controls the extent of heterogeneous growth. In this work, the growth of cadmium and cobalt carbonate phases on calcite surfaces is compared for a range of initial saturation states with respect to otavite (CdCO3) and sphaerocobaltite (CoCO3), two isostructural metal carbonates that exhibit different lattice misfits with respect to calcite (-4% and -15%, respectively, based on surface areas). Calcite single crystals were reacted in static conditions for 16h with CdCl2 and CoCl2 aqueous solutions with initial concentrations 0.3 less than or equal to [Cd2+]0 less than or equal to 100 mu M and 25 less than or equal to [Co2+]0 less than or equal to 200 mu M. The reacted crystals were imaged in situ with atomic force microscopy (AFM) and analyzed ex situ with X-ray photoelectron spectroscopy (XPS). AFM images of Cd-reacted crystals showed the formation of large islands elongated along the direction, clear evidence of heteroepitaxial growth, whereas surface precipitates on Co-reacted crystals were small round islands. Deformation of calcite etch pits in both cases indicated the incorporation of Cd and Co at step edges. XPS analysis pointed to the formation of a Cd-rich (Ca,Cd)CO3 solid solution coating atop the calcite substrate. In contrast, XPS measurements of the Co-reacted crystals provided evidence for the formation of a mixed hydroxy-carbonate cobalt phase despite supersaturation with respect to CoCO3. The combined AFM and XPS results suggest that the lattice misfit between CoCO3 and CaCO3 is too large to allow for heteroepitaxial growth of a pure cobalt carbonate phase on calcite surfaces in aqueous solutions and at ambient conditions. The use of the satellite structure of the Co 2p 3/2 photoelectron line as a tool for determining the nature of cobalt surface precipitates is also discussed.