Quantitative and structural analysis of minerals in soil clay fractions developed under different climate zones in China by XRD with Rietveld method, and its implications for pedogenesis

Mineral compositions and structures in soil clay fractions can reflect the pedogenesis and the pedoenvironments of the natural soils. However, the simultaneous quantification and structural analysis of all phases in soils by X-ray diffraction are difficult, mainly due to overlapping reflections. In this study, quantification and structural analysis of the mineral phases in clay fractions in five soils (Alliti-Udic Ferrosol, Claypani-Udic Argosol, and Hapli-Udic Argosol, Malan loess, and Paleosol), developed under different climate zones in China, were carried out by the Rietveld method. Before proceeding with these analyses for the natural soil clay fractions by the Rietveld method, this method was applied to a set of artificial soils of montmorillonite (including a 6.8% quartz impurity), kaolinite, goethite, hematite, and magnetite to evaluate the accuracy of the method. Moreover, in the Rietveld method, the structure phase models, background function, profile models, the initial values for the correction of preferred orientation, and sequence of operations in Rietveld refinement parameters were optimized to improve the accuracy of the method. The evaluation demonstrated the quantitative analysis by the Rietveld method can obtain relatively satisfied results. The absolute errors for the mineral contents below 5% were in the range of −0.49%-+0.63%, and those for the mineral contents above 10% were in the range of −6.32%-+5.00%. The absolute errors for the Al-substitutions in the goethite and hematite are −0.8 Al mol%-+3.2 Al mol% and −0.3 Al mol%-+3.3 Al mol%, respectively. Employing the above Rietveld method, the content of every mineral and Al-substituted iron (hydr)oxides (goethite and hematite) in the clay fractions in the five soils developed under different climate zones were obtained. In combined with the soil physicochemical properties, the order of pedogenic development of the five natural soils is Alliti-Udic Ferrosol > Claypani-Udic Argosol > Hapli-Udic Argosol and > Malan loess, respectively.