Effect of divalent cations on the formation and structure of calcium carbonate polymorphs

The incorporation of divalent cations (Mg2+, Mn2+, Cu2+, Sr2+, Cd2+, Ba2+ and Pb2+) in vaterite and calcite has been studied by atomic absorption spectroscopy (AAS), optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. The samples of vaterite and calcite were prepared by spontaneous precipitation from aqueous solutions under defined conditions. The particles of vaterite obtained were spheres, being aggregates of crystallites of 25-35 nm, and the crystals of calcite were well defined rhombohedra. All ions examined, except Cu2+, were found to be incorporated in vaterite probably forming solid solutions. Ba2+ caused a distortion of the vaterite crystal lattice and Mg2+ changed the shape of the particles. Mn2+ ions were incorporated into vaterite in at least two different environments, segregated into clusters or substituted for Ca2+, as detected by EPR. The environment of those Mn2+ ions substituted for Ca2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by gamma-irradiation of vaterite and calcite are presented. These spectra were used to detect formation of calcite in the vaterite system, in the presence of Cd2+ and Pb2+ ions confirming the results obtained by EPR of Mn2+ as a paramagnetic substitute for Ca2+. Free radicals characteristic of Cd2+ ions, were detected in both vaterite and calcite gamma-irradiated under similar conditions.