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Mode and Sites of Incorporation of Anions in Calcite

Calcite is of great interest because of its wide use both in industry (pigment and filler in paper, rubber, plastics, paints etc.) and in the natural environment (biomineralization, geological scales). It also has important application in energy production technology and water treatment. In all these instances the crystal size distribution and morphology of the crystals are of great significance. Additives and impurities of both inorganic and organic nature play here an important role. In the recent years, the most intensively investigated additives concerning this matter were soluble polymeric and special functional low molecular weight additives, as well as the protein macromolecules isolated from some living organisms. Among the inorganic ions and small molecules, the effect of Mg2+ and some organic acids on the formation and morphology of calcium carbonate polymorphs were much more studied than the effect of anions and other cations. In order to investigate the role of anions separately from the foreign cations in the calcium carbonate precipitation system, calcite was precipitated from a homogeneous system in which calcium hydroxide (ci(Ca2+) = 5.0  10-3 mol dm-3) and carbonic acid (ci(CO32-) = 9.0  10-3 mol dm-3) solutions were used as reactants. Thus, any possible effects of other ions than constituent ions (Ca2+, CO32-) and the products of autoprotolysis of water (H3O+, OH-) on the precipitation of calcite were avoided. Foreign anions were added to the precipitation system in the form of calcium salt (CaSO4, Ca(NO3)2 and CaCl2) solutions. The aim of this work was to shed more light upon mode and sites of the possible incorporation of these anions into the calcite lattice. For this purpose X-ray powder diffraction (XRD), FT-IR and electron paramagnetic resonance (EPR) spectroscopies, scanning electron microscopy (SEM), thermogravimetry and ionic chromatography were used. In order to study the local environment in calcite crystal lattice by EPR spectroscopy, Mn2+ was used as a paramagnetic substitute for Ca2+. It was found that the crystal size distribution and morphology were not affected by the addition of the co-anions in concentrations examined (1.0  10-4, 3.0  10-4, 6.0  10-4 and 1.0  10-3 mol dm-3). Nevertheless, the incorporation of the co-anions in the crystal lattice of calcite was detected. The calcite unit-cell parameters (i.e., a, b, c, and unit-cell volume) of all powder samples were measured and the disturbation of the calcite crystal lattice was found to increase in the series chloride>nitrate>sulphate. The results obtained will be discussed and a model proposed by which the mode and sites of the foreign anions incorporation in calcite can be explained.

Anions; Calcite; Crystal growth; Sulphate

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