EPR, FT-IR and XRD investigation of soils from Paraná, Brazil

Samples of Araucária area soil from Paraná state, Brazil, were separated by particle size fractionation and investigated by electron paramagnetic resonance (EPR) in X-Band of 9.5 GHz at room temperature and 77 K, infra-red spectroscopy and X-ray diffractometry. The paramagnetic species in the soil samples were identified by comparison with EPR spectra of some minerals studied recently by our group, several soil types and/or soil components investigated in the literature. The value of g = 2.1 (ΔH = 85 mT) indicated the presence of ferrihydrite. Hematite was identified by g = 2.1 (ΔH = 100 mT) and g = 4.3 for Fe3+ lines of the concentrated dominium and diluted dominium. Kaolinite was identified by IR and EPR with the resonance at g = 4.3 attributed to Fe3+ ions in isolated sites of tetrahedral and octahedral symmetry with rhombic distortion. The resonances at g = 3.7 and g = 4.9 were attributed to Fe3+ in more highly symmetrical environment than rhombic symmetry, but not in axial symmetry. Three signals around g = 2 were attributed to radiation defects, plus additional resonances at g = 2.8 and 9.0. Signals less intense than those at g = 2.1, 3.7, and 6.5, observed for clear grains of soil, were attributed to presence of Fe3+ in quartz which was identified by IR and XDR.

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► The investigated Brazilian soils presented the EPR signals of ferrihydrite and hematite.
► Kaolinite was identified by IR and EPR.
► Defects in the kaolinite structure were detected by the EPR spectra.
► Less intense EPR signals were attributed to presence of Fe3+ in quartz.