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Crystallization of Calcite in the Presence of Mg2+ and Inorganic Anions

Calcium carbonate is widespread on earth as geological and biological material and it is also one of the most important raw materials for use as filler and pigment in a variety of industrial productions. Such a diversity of finding places, roles in natural processes and uses in industrial production, demand different approaches to the investigation of this material. In this work the influence of Mg2+ and some inorganic anions, common impurities in any natural and industrial precipitation systems, on the physical and chemical properties of calcite was investigated. Calcium carbonate was precipitated from calcium hydroxide and carbonic acid solutions at 25 °C, with and without the addition of different magnesium (MgSO4, Mg(NO3)2 and MgCl2) and sodium salts (Na2SO4, NaNO3 and NaCl) of identical anions. In order to study morphology, crystal size distribution, composition, structure and the specific surface area of the precipitated crystals, as well as the mode of cation- and anion-incorporation in the calcite crystal lattice, a combination of optical and scanning electron microscopy (SEM), electronic counting device, multiple B.E.T. method, thermogravimetry, FT-IR spectroscopy, X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy were used. It was found that calcite was the first and the only polymorphic modification of calcium carbonate that appeared in the precipitation systems of lower initial relative supersaturations. In the systems of high initial supersaturations, precipitation of an amorphous precursor phase preceded the formation of calcite. By increasing the magnesium concentration in solution, the content of magnesium in calcite also increased and showed an evident correlation with the type of magnesium salt used. Incorporation of magnesium caused the formation of crystals elongated along the calcite c-axis and, in some cases, the appearance of new {; ; 011}; ; faces. The formation of polycrystalline aggregates occurred on increasing the ci(Mg2+)/ci(Ca2+) molar ratios in solution. The addition of sulphate ions alone caused the formation of spherical calcite polycrystalline aggregates.

Crystallization; Calcite; Magnesium ion; Inorganic anions

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