57Fe Mössbauer, XRD, FT-IR, FE SEM Analyses of Natural Goethite, Hematite and Siderite

Abstract

Natural goethite, hematite and siderite were analysed with 57Fe Mossbauer, XRD and FT-IR. FE SEM images of samples were also taken. The Mossbauer spectra of limonite (α-FeOOH · nH2O) from Budapest (Hungary), Ljubija (Bosnia and Herzegovina) and Korcё (Albania) showed the same type of spectrum, indicating low crystallinity and broad particle size distribution. All goethite particles from these three locations were one-dimensional (1D), but with different nano/microstructures. A very early precursor of limonite from Budapest and Ljubija locations was assigned to FeS2 (pyrite and/or marcasite) which oxidised upon ventilation (oxygenation) under hydrogeothermal conditions, thus producing FeSO4 and Fe2(SO4)3. In the next step limonite deposits were formed. The similarity between this limonite formation under hydrogeothermal conditions and the chemical precipitation of goethite from FeSO4 or Fe2(SO4)3 solutions at laboratory level was briefly discussed. The deposition of lateritic goethite at the Korce location is presumed to be due to the chemical weathering (tropical conditions) of ultramafic rocks. Under the same conditions and a proper pH the transformation of goethite to hematite is possible. Alternatively, the oxidation of Fe2+ in magnetite and its transformation to hematite via maghemite (γ-Fe2O3) as an intermediate could have taken place. The Mossbauer spectrum of siderite from the Ljubija location showed a quadrupole doublet with asymmetric spectral lines. This asymmetry could be assigned to the Goldanskii-Karyagin effect, however, the contribution of the crystallite texture to this asymmetry cannot be excluded. Hematite and a small fraction of siderite at the Vares location (Bosnia-Herzegovina) are of metasomatic origin deposited in limestone that now form a series of greatly metamorphosed sedimentary rocks. Hematite particles were deposited in the form of laminates (2D).