X-RAY-DIFFRACTION, INFRARED AND MOSSBAUER STUDIES OF FE-RICH CARBONATES

Siderites and Fe-rich members of the dolomite-ankerite series have been studied by X-ray diffraction, FTIR and Fe-57 Mossbauer spectroscopy. X-ray diffraction data indicate a clear variation in unit-cell parameters with composition, although carbonate compositions close to pure siderite show some scattering. Infrared spectroscopy displays small shifts (<10 cm-1) in the three vibrational frequencies which appear near 1420 cm-1 (nu-3), 870 cm-1(nu-2) and 730 cm-1(nu-4). The relationship between the studied carbonate compositions and the bands at 870 cm-1 and 730 cm-1 is approximately linear for siderites, whereas for ankerites this is non-linear. Mossbauer data reveal the presence of one unique type of site occupancy as well as certain distortion in FeO6 octahedra in siderites. Quadrupole splitting (QS) and Fe content seem to be independent from each other, but certain correlation between the amount of Fe and the band area exists. The variations observed for compositions close to pure siderite may be possibly due to the antagonic effect caused by the substitutions of Ca and Mg for Fe in octahedral sites.