Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by BEERMANN, T. Articles by BROCKAMP, O. Search for Related Content GeoRef GeoRef Citation

Structure analysis of montmorillonite crystallites by convergent-beam electron diffraction

Fachgebiet Mineralogie, FB-5, Universität Bremen, Postfach 330 440, D-28334 Bremen, Germany

^* E-mail: michelse{at}min.uni-bremen.de

(Received 26 August 2003; revised 12 May 2004)

The small particle size and the random stacking of layers has previously hindered systematic structure investigations of montmorillonite. By applying the convergent-beam electron diffraction mode (CBED) of a transmission electron microscope (TEM) with a beam spot of ~800 Å we were able to examine undisturbed areas of montmorillonite crystallites.

Because montmorillonite crystallites are mostly thin particles, kinematic theory can be applied and the CBED patterns can be interpreted directly, provided that the particle thickness remains below the critical value of 350 Å. An average thickness of ~90 Å was calculated here for montmorillonite of bulk samples from X-ray diffraction analysis and lattice-fringe images. However, satisfactory diffraction intensity patterns for quantitative evaluation were obtained only from crystallites with a thickness above the average, which yielded a sufficient scattering volume. These patterns could be described in terms of the kinematic theory and therefore these crystallites were <350 Å thick. Yet, crystallites of adequate thickness were extremely rare in the three samples investigated (Clay Spur, Rock River and Upton, all in Wyoming, USA).

The diffraction intensities from the ab plane of single montmorillonite crystallites of the various origins fit the three structural models for a trans-vacancy distribution, a cis-vacancy distribution or a random-cation distribution within the octahedral sheets. The configuration of the diffraction patterns also shows a 1M symmetry of the layer. Due to the limited data set of CBED patterns, a refinement of the structure could not be achieved. However, energy dispersive X-ray spectroscopy data and computation of the cation–anion distances and valences using the ‘distance valence least square’ program permitted a refinement of the models.

KEYWORDS: montmorillonite, structure analysis, electron diffraction, CBED