Rietveld Analysis / Whole Pattern Fitting Method for Quantification of Clay Components

This study explores the use of Rietveld Refinement and Whole Pattern fitting as an alternative technique for quantitative clay analysis. Rietveld Refinement uses known atomic structure to model the phase components present. This allows the refinement of atomic parameters through least squares analysis until the calculated pattern, generated by the model, agrees with the experimental pattern. Rietveld analysis can yield accurate and reproducible quantitative results. The traditional methods are highly subjective to variations in diffracted peak intensity due to preferred orientation. This is not a limitation in the Rietveld method.

To use Rietveld analysis for both the expandible and non-expandible clays, the atomic positions are necessary to generate a combined atomic structure model . While the starting atomic positions of the expanded lattice are difficult to determine, it's experimental diffraction pattern can be readily measured for the base lattice phase. Then, using the least square refinement to generate a simulated crystal structure of the expanded system, the expandible clay structures can be simulated. This new expanded clay model then becomes the starting atomic model for Rietveld Refinements and can quantifying the expandable specimen component. With a fairly accurate atomic model of montmorillionite and illite, a series of quantitative mixtures were analyzed for accuracy and precision.