Soil characterisation using X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance

The optical absorption spectra and chemical composition of soil samples were characterised using photoacoustic spectroscopy (PAS), electron paramagnetic resonance (EPR), X-ray diffraction and X-ray fluorescence. From fluorescence results, the chemical components were identified and an Fe mass concentration varying between 4% and 10% was determined. Besides that, the observed photoacoustic technique (PA) spectra showed transition bands associated with Fe3+ ions. From the phase behaviour, both the nonradiative relaxation time τ and the characteristic thermal diffusion time τβ were determined. The X-ray diffraction analysis showed that kaolinite is a major crystalline phase (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that the Fe3+ cations partially substitute for Al3+ cations in the octahedral sites of the kaolinite structure. EPR measurements were performed in order to determine the crystalline environment of Fe ions; the observed profiles indicate that Fe sites are embedded in a distorted cubic crystalline field.