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

The incorporation of divalent cations (Mg^2+, Mn^2+, Cu^2+, Sr^2+, Cd^2+, Ba^2+ and Pb^2+) in vaterite and calcite was 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 Cu^2+, were found to be incorporated in vaterite probably forming solid solutions. Ba^2+ caused a distortion of the vaterite crystal lattice and Mg^2+ changed the shape of the particles. Mn^2+ ions were incorporated into vaterite in at least two different environments, segregated into clusters or substituted for Ca^2+, as was detected by EPR. The environment of those Mn^2+ ions substituted for Ca^2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by gama-irradiation of vaterite and calcite are presented. These spectra were used to detect formation of calcite in the vaterite system, in the presence of Cd^2+ and Pb^2+ ions confirming the results obtained by EPR of Mn^2+ as a paramagnetic substitute for Ca^2+. Free radicals characteristic of Cd^2+ ions, were detected in both vaterite and calcite gama-irradiated under similar conditions.

Calcium carbonate ; Vaterite ; Calcite ; Effect of divalent cations ; EPR spectra

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