Comparison of methods for the quantification of montmorillonite in bentonites

In this paper, we describe and evaluate three methods for measuring the amount of montmorillonite in 10 bentonite samples. Each of these methods is based on certain assumptions that usually result in analyses of low accuracy. In order to assess the attainable accuracy, two procedures for the determination of montmorillonite contents are tested. The first is based on the quantitative separation of the <2-μm fraction in combination with XRD-analysis. The second considers cation exchange capacity (CEC), layer charge (LC) and variable charge. In addition, a third and new method based on the comparison of the CEC of a bentonite with the CEC of the respective pure montmorillonite fraction is proposed. This new method provides accurate values for montmorillonite contents, but is restricted to bentonites, which are free of X-ray amorphous fine-grained constituents. By combining the results of all methods, the layer charge (charges/half unit cell (HUC)) was calculated. Obtained results are comparable with the well-known n-alkylammonium method.