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

Abstract

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 γ-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 γ-irradiated under similar conditions.