Routine investigation of structural parameters of dioctahedral smectites by the Rietveld method

Abstract

Abstract X-ray powder diffraction (XRD) in combination with the Rietveld method is a common technique for crystal structure analysis. However, almost all minerals of the smectite group show turbostratic disorder preventing the direct application of Rietveld refinement. Ufer et al. (2004) proposed to overcome this limitation by using a single layer model with the BGMN program. This model is now routinely used for quantitative phase analysis of smectite-containing samples. However, the refinement of some important structural features, such as the layer charge density (LCD) and the octahedral iron content has not been achieved successfully yet. Typically, these structural features can be obtained only by analytical methods which require purified smectite samples. The project was firstly aimed to prove the validity of the refinement of selected structural parameters by using a set of purified and well-characterized samples and then applied this method to raw bentonites with natural impurities. The Rietveld refinements were carried out on the X-ray powder diffraction patterns of the Cu-triethylenetetramine (Cutrien) exchanged dioctahedral smectites to avoid the problems of unknown cation species and complex hydration behavior in the interlayer. The refinements of the occupancy of octahedral iron and the LCD led to reasonable results for iron-poor purified samples. The application of the structure model to Cutrien -exchanged smectite-rich samples with natural impurities, e.g. bentonites without previous purification, showed that some important structural details of turbostratically disordered dioctahedral smectites can be determined by the Rietveld method from XRD patterns of the Cutrien -exchanged samples. The technique may be applied in routine structural characterization of raw smectites in bulk rock samples without a prior chemical analysis of purified smectite fractions.