Quantitative phase analysis of natural raw materials containing montmorillonite

One of the limits of quantitative phase analysis of natural clays using the Rietveld method is the lack of an applicable structure model for the highly disordered phases such as montmorillonite. The description of disorder of montmorillonite, the main dioctrahedral smectite, was accomplished using the DIFFaX program [1] which allows the calculation of simulated powder patterns. We followed the strategy successfully used for kaolinite [2] in which simulated powder patterns calculated using different models of disorder are compared to the observed one. Since DIFFaX is bases upon a recursive method and produces a statistical crystal, a super-cell composed of n layers describing the average disordered structure was re-defined to be used in the codes which apply the deterministic Rietveld algorithm. Therefore, a working structure composed of 4 layers (an ideal layer, a b/3 shifted layer, a -b/3 shifted layer, and a -a/3 shifted layer) was described and used in GSAS [3] to fit a natural international standard of Ca-montmorillonite.