Rietveld refinement of structural parameters of dioctahedral smectites

Turbostratically disordered structures like smectites can be treated within the Rietveld method by a supercell approach. However, the refinement of some important structural features, such as interlayer site occupation as a measure for layer charge density, proportion of cis- and trans-vacant layers and the octahedral site occupancy by iron have not been done successfully yet. A structural model containing the atomic coordinates of 2:1 layers and an interstratified structure consisting of cis- and trans-vacant layers was chosen. The Rietveld refinement was primarily carried on the simulated patterns. The same model was applied on purified montmorillonite and beidellite samples. A specific model was used for purified nontronite samples owing to the high iron content. Finally, these two structural models were applied on smectite-rich samples with natural impurities, e. g. bentonites without previous purification. All samples were exchanged with Cu-triethylenetetramine to avoid the problems of unknown cation species and complex hydration behavior in the interlayer.