Quantitative clay mineralogy of several Louisiana soils

A method was developed to simplify the approach to obtaining a better quantitative representation (QR) of complex clay mineral assemblages by XRD analysis. The method has eight steps: 1) preparation of samples with preferred orientation; 2) qualitative identification of phases; 3) peak decomposition to obtain precise locations of individual peaks contained within broad diffraction bands; 4) creation of a library of theoretical XRD patterns by modeling of one-dimensional XRD patterns; 5) selection of experimental library patterns; 6) employing the LINEST function found in common spreadsheet software to fit library patterns to experimental patterns; 7) evaluation of results to determine needed improvements; 8) fine-tuning of results by manual adjustment to contribution coefficients. Results obtained from the Kenner, Schriever, Mahan, and Ruston soil series indicate that the method is capable of generating improved QR data. Duplicate results were typically within 10% RSD. Those components that did not replicate at better than 10% RSD were typically present at less than 10% by weight. Results show high percentages of mixed-layer clay minerals present in all samples; between 16% and 57% by weight of crystalline material. The most common mixed-layer clay mineral was illite/smectite followed by illite/vermiculite and illite/chlorite. Correlation to CEC and ECEC measurements indicate that the method is capable of generating plausible mineral assemblages, based on good agreement between calculated composite exchange capacity values and measured exchange capacity values, and is able to differentiate between dissimilar mineral assemblages, reasonably able to differentiate between similar mineral assemblages, and in many cases able to distinguish similar sections within the same profile from one another