Thirty six bentonite samples from 16 different locations were examined in order to demonstrate the applicability of a new Rietveld description approach for quantitative phase analysis. X-ray diffraction patterns of the bulk material were obtained and analyzed by the Rietveld method. The samples contain up to ten different minerals, with dioctahedral smectite as the major component. A model for turbostratic disorder of smectites was formulated inside a structure-description file of the Rietveld program BGMN. The quality of the refinements was checked using an internal standard mineral (10.0 or 20.0 wt.% corundum) and by cross-checking results with X-ray fluorescence (XRF) data. The corundum content was reproduced with only small deviations from the nominal values. A comparison of the chemical composition obtained by XRF and the composition as re-calculated from quantitative Rietveld results shows a satisfactory agreement, although X-ray amorphous components such as volcanic glasses were not considered. As a result of this study, the Rietveld method combined with the new structure model for turbostratic disorder has proven to be a suitable method for routine quantitative analysis of bentonites with smectites as the dominant clay minerals.

The cation exchange capacity (CEC) is one of the most important properties of clays in terms of their performance in both natural and technical processes. For decades, common methods for determining exchangeable cations have failed when calcareous clays or soils were examined, because calcite is at least partly dissolved throughout the exchange experiment which in turn increases measureable Ca²⁺ concentrations. As a result, exchangeable Ca²⁺ and the sum of exchangeable cations by far exceed the amount of negative charges. In the past, the silver-thiourea method (AgTU) has been modified to overcome this problem (AgTU_calcite), but is unsatisfactory as the method is laborious. In the present study three new methods based on two alternative metal complexes, cobalt(III) hexamine (CoHex) and copper(II) triethylenetetramine (Cu-trien), are proposed. The optimum solid/liquid ratios of these methods and the optimum complex concentration of Cu-trien are reported, depending on the mineralogical composition of the samples. The key development is that the exchange solutions are saturated with respect to calcite prior to the experiment. Approximately 70–90% of the dissolution of calcite present as an admixture in a clay sample is suppressed in the subsequent cation exchange experiment, but not all. The Ca²⁺ exchange is not suppressed and there is no evidence for any precipitation of this Ca²⁺. Three possibilities for how to handle this problem are discussed, one of which is to perform no further correction. The resulting error arises from the remaining calcite solubility of the different solutions after pre-treatment with calcite. This corresponds to errors of 0.2–1.3 (CoHex_calcite)and 0.7–8.4 (Cu-trien_calcite) meq/100 gCa²⁺ for samples with small and large CEC values, respectively. As a consequence of the poor performance of Cu-trien_calcite for samples with large CEC, a more concentrated Cu-trien_{5 × calcite} solution was developed which performed much better: 0.1–0.8 meq/100 g(Cu-trien_{5 × calcite}). For Cu-trien_{5 × calcite} and CoHex_calcite at least, the errors are in the range of the non-systematic scattering for exchangeable Ca²⁺ determination. Therefore, the methods suggested provide ‘operationally correct’ Ca²⁺ values without additional effort. Moreover, owing to the high selectivity of the index cations applied in the present study, only one exchange step is required, providing a significant advantage over the AgTU_calcite method.

The pH of aqueous bentonite suspensions is known to be influenced by carbonates present even in minor amounts. On the other hand, at high solid:liquid ratios (at standard pH measurement conditions: 2% w/w suspension), the type of exchangeable cation in the smectite is also known to determine pH (particularly Na⁺ or Ca²⁺). By cation-exchange tests we proved that exchanging the Ca²⁺ for Na⁺ results in an increase in the pH. However, this increase in pH was only found if excess salts were removed from the system (by washing or dialysis, respectively). The effect of the type of exchangeable cation can, at least partially, be explained by hydrolysis of Ca²⁺. On the other hand, a pronounced alkalinity of Na bentonites is observed which can, at least partially, be attributed to the hydrolysis of montmorillonite (Na⁺ is exchanged for H⁺ of water). The increase in the volume of the Stern layer, caused by increasing the degree of delamination, is also suggested to play a role. H⁺ and Na⁺ are concentrated in the Stern layer. Hence, increasing the Stern layer volume decreases the amount of H⁺ and Na⁺ in solution and thus increases pH. Unfortunately, both processes, montmorillonite hydrolysis and delamination, depend on the ionic strength. Distinguishing the processes quantitatively, therefore, is an analytical challenge, and impossible based on the data presented here.To model the pore-water chemistry of clays and clay stones, all of the above-mentioned processes have to be considered. It is possible that other reactions, not identified in the present work, contribute toward the pH values of aqueous bentonite suspensions.