Quantitative phase analysis of clay soils via the Rietveld-RIR method and thermal analyses coupled with evolved gasses mass spectrometry

The present study deals with Quantitative Phase Analysis (QPA) of clay soils obtained via the Rietveld-RIR method, the General Structure Analysis System (GSAS; Larson & Von Dreele, 2000), and thermal analyses coupled with evolved gasses mass spectrometry. As far as we know, there are very few applications of the Rietveld method to the estimation of quantitative mineralogical composition of soils (Brinatti et al., 2010; Prandel et al., 2014). The application of the Rietveld method represents a major step forward in QPA with respect to conventional methods, especially as far as accuracy and detection limits are concerned. The GSAS is a comprehensive and detailed system for refinement of both structural models and quantitative mineralogical composition of powder mixtures, starting from X-ray or neutron diffraction data. Thermogravimetry (TGA) allows the determination of the mass as a function of temperature or time; this thermal technique provides information concerning thermal stability and composition of the sample and of any intermediate compound which may be formed. When these measures are combined with chemical analysis of evolved gases, it is possible to determine the reactions that are at the base of each thermal effect. During the GSAS refinements, a Chebyshev polynomial of the first kind was used for background modeling. For peak shape modeling a multi-term Simpson's rule integration of pseudo-Voigt was adopted. The amount of amorphous phase present in the analyzed powder mixtures was estimated via the combined Rietveld and Reference Intensity Ratio (RIR) methods. Corundum NIST SRM 674a was used as internal standard. Identified phases in analyzed soil samples include tectosilicates, layer silicates, carbonates, and in minor amount sulphates. Irregular interstratified phyllosilicates were also detected. Since treatments with ethylene glycol did not enhanced any appreciable changes in the low angle peaks of the pattern, it is conceivable that no expandable interstratified phyllosilicates are present in the analyzed soils. One or more amorphous phases were detected. In selected samples the amount of carbonates and hydrated phases measured via X-ray diffraction was then compared with the values estimated via TGA by measuring the quantity and type of the reaction products (i.e., H2O and CO2) released. Brinatti A.M., Mascarenhas Y.P., Pereira V.P., Partiti C.S.d.M. & Macedo A. 2010. Mineralogical characterization of a highly-weathered soil by the Rietveld Method. Scientia Agricola, 67(4), 454-464. Larson A.C. & Von Dreele R. 2000. General Structure Analysis System (GSAS). Los Alamos National Laboratory Report LAUR 86-748. Prandel L.V., Saab S.C., Brinatti A.M., Giarola N.F.B., Leite W.C. & Cassaro F.A.M. 2014. Mineralogical analysis of clays in hardsetting soil horizons, by X-ray fluorescence and X-ray diffraction using Rietveld method. Radiat. Phys. Chem., 95, 65-68.