EFFECT OF DIVALENT-CATIONS ON THE FORMATION AND STRUCTURE OF CALCIUM-CARBONATE POLYMORPHS

The incorporation of divalent cations (Mg2+, Mn2+, Cu2+, Sr2+, Cd2+, B a2+ and Pb2+) in vaterite and calcite has been studied by atomic absor ption spectroscopy (AAS), optical and scanning electron microscopy (SE M), X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. The samples of vaterite and calcite were prepared by spo ntaneous 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 defin ed rhombohedra. All ions examined, except Cu2+, were found to be incor porated in vaterite probably forming solid solutions. Ba2+ caused a di stortion 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 fo r Ca2+, as detected by EPR. The environment of those Mn2+ ions substit uted for Ca2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by gamma-irradiatio n of vaterite and calcite are presented. These spectra were used to de tect formation of calcite in the vaterite system, in the presence of C d2+ 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 und er similar conditions.