The use of the Rietveld Method (RM) with X-ray diffraction data (XRD) in soils becomes difficult in relation to the best adjustment and higher reliability of the mineralogical quantification. One of these difficulties is related to the mineral phase starting crystalline structures, once they derive from natural samples. Therefore, it is necessary to seek starting structures of the mineral present in the soil closer to that found in Brazilian soils. This study analyzed samples coming from the hardsetting of a Yellow Latosol (Oxisol), Pará State. The procedure consisted of separating the mineral Goethite from the remaining minerals present in the clay fraction, through chemical treatment. The clay fraction was centrifuged and a very fine clay fraction was obtained from the supernatant which was possible to identify, through XRD, the minerals Goethite and Kaolinite and, through chemical treatment, Kaolinite was eliminated. The XRD enabled to identify the minerals Goethite, Analcime, Muscovite and Sodalite in the extraction product. In this chemically treated sample, the RM refinement was carried out and the Goethite structure was obtained. In the sand fraction sample, it was possible to identify, through XRD, the minerals Quartz and Goethite, and through RM the quartz structure presents in this fraction was refined by employing the previously refined Goethite structure. XRD analyses with CuKα radiation, in the step-by-step mode and extension and the RM refinements were carried out employing the GSAS.
This work aimed to employ differential scanning calorimetry to find the possible desidroxylation/amorphization temperatures of the main minerals composing a particular soil clay fraction, as well as characterize this fraction mineralogically through XRD at several temperatures. The clay fraction comes from the hardsetting horizon of a Yellow Latosol (Oxisol) formed from sandy and sandy-clayey non-consolidated sediments from the Barreiras formation (Pará State). Considering the soil solid phase, composed of organic and mineral matter, with several and specific physical and chemical properties, it becomes difficult to identify minerals directly only using the X-ray diffraction, since due to their amount, many diffraction peaks overlap. Thus, the samples thermal treatment represents a good alternative to help identify the crystalline mineral phases present in the sample. The clay fraction sample was collected by the Stokes Law. The DSC analyses were used with 30 mg sample heated from 25 °C to 800 °C at 10 °C/min rate and a 20 mL/min Argon flow. The XRD analyses were carried out using a diffractometer with CuKα radiation; in 2°/ min continuous scanning and 2θ from 3° to 100° extension, with an oven coupled to the diffractometer for heating at 200 °C, 300 °C, 400 °C, 470 °C, 510 °C, 600 °C, 700 °C and 800 °C. The disappearance of goethite peaks at 400 °C and kaolinite peaks at 510 °C were observed through the XRD analysis, in accordance with the DCS analyses whose temperatures corresponding to the endothermal peaks were 328 °C and 516 °C, respectively, so that the use of such analyses helped to clarify the mineral composition of the clay fraction under analysis.
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