engleski

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 (SO42-, NO3- and Cl-) in the calcite crystal lattice. Calcite, the thermodynamically stable calcium carbonate polymorph at 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 calcium salt solution, before mixing the reactants. In order to study the local environment in calcite crystal lattice by EPR spectroscopy, Mn2+ was used as a paramagnetic substitute for Ca2+. The experiments were performed at 25 °C and the samples for analyses were taken after a predetermined aging time of 20 min, when the reaction was stopped. It was found that the co-anions, when added in the concentrations examined, did not affect the morphology of calcite crystals but they incorporated into the calcite structure. The largest distortion of the calcite structure was obtained when sulphate ions were added. The measured parameters of the calcite unit-cell are given, the results obtained are discussed and a model that explaines the mode and sites of sulphate incorporation is represented.

anions; calcite; calcium carbonate; crystal growth; EPR spectroscopy

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