Incorporation of inorganic anions in calcite

A number of analytical methods and techniques (X-ray diffraction, FT-IR and electron paramagnetic resonance spectroscopy, ionic chromatography, scanning electron microscopy, etc.) have been used to study the mode and sites of incorporation of some common inorganic anions (S0(4)(2-), NO3- and Cl-) in the calcite crystal lattice. Calcite, the thermodynamically stable calcium carbonate polymorph under standard conditions, was prepared by spontaneous precipitation from calcium hydroxide and carbonic acid solutions. The particular co-anion was added into the carbonic acid solution in the form of a calcium salt before mixing the reactants. The system prepared in such a way is particularly suitable for investigating the mode of foreign anion incorporation into the crystal lattice of calcite. Apart from the constituent ions and the products of water protolysis, this system contains only the co-anion examined. In order to study the local environment in the calcite crystal lattice by EPR spectroscopy, Mn2+ was used as a paramagnetic substitute for Ca2+. The experiments were performed at 25degreesC and the samples for analyses were taken after a predetermined ageing time of 20 min. It was found that the co-anions, when added in the concentrations examined, did not affect the morphology of the calcite crystals but were incorporated into the calcite structure. The largest distortion of the calcite structure was obtained when sulfate ions were added. The measured parameters of the calcite unit cell are given, the results obtained are discussed and a model that explains the possible mode and sites of sulfate incorporation is presented. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.