Quantitative analysis of X-ray diffractograms of turbostratically disordered smectites with an approach suitable for Rietveld-refinement

Country / Region: Germany

Begin of project: January 1, 2001

End of project: March 1, 2005

Status of project: March 1, 2005

Figure 1: Schematic representation of a turbostratically disordered crystallite Source: redrawn after MOORE & REYNOLDS, 1997, ISBN 0195087135

The assessment of bentonite quality for industrial applications requires quantitative mineral analyses of smectite, their major clay mineral. The Rietveld method (Rietveld, 1967) which is successfully used in X ray powder diffraction for “normal” minerals, poses severe problems when dealing with smectites. Smectites often show turbostratic stacking faults (Fig. 1), which lead to an extremely asymmetric peak broadening. This broadening effect prevents sofar the use of the standard Rietveld-method to determine smectite contents.

Turbostratic disorder of layer structures can be explained as a random rotation and/or translation of the individual layers relative to each other. One of the ways to deal with the turbostratic effect is to use the Debye formula (Debye, 1915). This formalism allows to calculate a powder diffractogram from any arrangement of atoms without the need for translational symmetry and is therefore suitable for the simulation of all kinds of structural defects. Yang & Frindt (1996) developed an extension of the Debye formula to calculate the diffraction of turbostratical disordered structures in a very effective way. The application of the extended Debye formula showed that the turbostratic effect can be regarded as diffraction from an ordered one-dimensional crystal to account for the (00l)-reflections combined with the contribution from a single layer to represent the hkl-reflections with h and k ≠ 0.

These results were used to develop a realstructure model of smectites which is suitable for being refined by the Rietveld method using one of the available, very sophisticated program codes like BGMN (Bergmann et al., 1998). We demonstrate the applicability of this approach on a reference sample.

The PhD thesis of Dr. Ufer was honoured 2005 with the Karl-Jasmund-Preis of the German Clay Group DTTG (Deutsche Ton- und Tonmineralgruppe).

Literature:

• Ufer, K., Roth, G., Stanjek, H., Dohrmann, R., Kleeberg, R., Bergmann, J. 2004. Descprition and quantification of powder X-ray diffractograms of turbostratically disordered layer structures with a Rietveld compatible approach.- Z. Kristall. 219: 519-527
• Ufer, K., Stanjek, H., Roth, G., Dohrmann, R., Kleeberg, R.,Kaufhold, S. 2008. Quantitative phase analysis of bentonites with the rietveld method. Clays and Clay Minerals 56 (2), 272-282.
• Ufer, K., Kleeberg, R., Bergmann, J., Curtius, H., Dohrmann, R. 2008. Refining realstructure parameters of disordered layer structures within the Rietveld method, Zeitschrift für Kristallographie Supplements, 27, 151-158

Partner:

• Dr. K. Ufer (Promotionsprojekt), Forschungszentrum Jülich
• Prof. Dr. R. Roth, Institut für Kristallographie, RWTH Aachen
• Dr. R. Kleeberg, Institut für Mineralogie, TU Bergakademie Freiberg

• Prof. Dr. H. Stanjek, Institut für Mineralogie und Lagerstättenlehre, RWTH Aachen