Stabilization of the vaterite structure in the presence of copper(II): Thermodynamic and spectroscopic studies

The crystalline transformation of vaterite in the presence of copper(II) [C u(II)] has been investigated in ultrapure water and at room temperature usi ng several techniques; scanning electron microscopy (SEM), infrared (IR) an d micro-Raman spectroscopy. Our findings show that a slowing down of this t ransformation occurs and is intimately related to the generation of CuCO3 l ayers on CaCO3 surfaces. This phenomenon is also accompanied by a decrease of vaterite solubility. All these observations have led us to suggest that these Cu(II) coatings prevent the dissolution of the metastable calcium car bonate, vaterite. To prove this suggestion, experiments on solubilities of Cu2+-vaterite solids were performed and interpreted in terms of thermodynam ic equilibrium and stoichiometric saturation concepts. Thermodynamic and sp ectroscopic data demonstrate that the apparent stabilization of vaterite in the presence of Cu(II) is strongly dependent on the existence of water mol ecules in the lattice of the solid solution CuxCa1-xCO3 as (H2O)(y)CuxCa1-x CO3. The chemical composition of this complex has been determined by X-ray photoelectron spectroscopy and thermogravimetry. Thus, the free energy of f ormation of such hydrated Cu(II) complexes has been found to be slightly lo wer than that for calcite. However, for high Cu(II) concentrations, metalli c multilayers cannot grow indefinitely, and malachite [Cu-2(OH)(2)(CO3)] ap pears as a new phase. SEM and micro-Raman techniques have allowed successfu lly monitoring these morphological phenomena/transformation.