Improving Rietveld-Based Clay Mineralogic Quantification of Oxisols Using Siroquant

Although mineralogic quantitative phase analysis (QPA) of the soil clay fraction can provide useful information for the improvement of soil management practices, QPA often requires a combination of several analytical techniques, which can be expensive and time consuming. One alternative that involves a single analysis to give accurate QPA of soils is the use of the Rietveld method to analyze powder x-ray diffraction (XRD) data. In this study, we evaluated the accuracy of the XPD-Rietveld approach for mineralogic quantitative analyses of Oxisol clays when observed structure factors [F(hkl)] of pedogenic minerals (i.e., kaolinite, hematite, and goethite) are used in the Rietveld analyses performed using Siroquant software. The results showed that although the structures of disordered clay minerals are especially difficult to incorporate in standard Rietveld refinement, which relies on ordered three-dimensional structure models, Mineralogic quantification can be accurately done for complex matrices having a large number of phases and various degrees of structural and compositional disorders when observed F(hkl) values arc used. It is also possible to develop observed F(hkl) values for Al-hematite and Al-goethite from impure samples of such oxides to use as starting structure models for Rietveld analysis. We believe that this approach can be successfully extended to other geologic materials.