2 results
High-Resolution Transmission Electron Microscopy and Electron Diffraction of Mixed-Layer Illite/Smectite: Experimental Results
- David R. Veblen, George D. Guthrie, Jr., Kenneth J. T. Livi, Robert C. Reynolds, Jr.
-
- Journal:
- Clays and Clay Minerals / Volume 38 / Issue 1 / February 1990
- Published online by Cambridge University Press:
- 02 April 2024, pp. 1-13
-
- Article
- Export citation
-
High-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments have been performed on R1 and R> 1 illite/smectite (I/S) samples that from X-ray powder diffraction (XRD) experiments appear to contain well-ordered layer sequences. The HRTEM images confirmed earlier computer image simulations, which suggested that periodicities due to I/S ordering can be imaged in TEM instruments of moderate resolution. The experiments also confirmed that in instruments of this sort, the strongest contrast arising from the compositional difference between I and S layers occurs under rather unusual imaging conditions of strong overfocus. Some selected-area electron diffraction (SAD) patterns showed additional diffraction spots consistent with R1 and R3 ordering. SAD patterns and cross-fringes arising in HRTEM images from non-00l reciprocal lattice rows indicated that the stacking vectors of most adjacent 2:1 layers were not randomly oriented with respect to each other. Thus, the I/S was not fully turbostratic, but instead consisted of very thin, coherently stacked crystallites that extended across the fundamental particles postulated by Nadeau and coworkers.
S/(I + S) ratios were determined for about seventy HRTEM images obtained and interpreted by three different TEM operators. These ratios were consistent with those obtained from standard XRD procedures, suggesting that results obtained by XRD can be used to infer the initial structural state of mixed-layer I/S prior to treatment of samples for XRD experiments. The HRTEM experiments thus demonstrated that the two specimens examined consisted of ordered I/S existing as small crystals, most of which contained more layers than the fundamental particles of Nadeau and coworkers. The non-turbostratic stacking suggests an energetic interaction between the individual fundamental particles, leading to at least two alternative thermodynamic descriptions of these materials. Although the I/S crystals in the present experiments probably were disaggregated into fundamental particles during sample preparation for XRD, the I/S crystals appear to have separated only along the smectite interlayers. If the term “fundamental particle” is to be used for primary, untreated I/S, its original definition should be modified to include not only free particles, but also those that occur as layers within small crystals. It further should be recognized that these particles can interact thermodynamically and crystallographically with their neighbors.
High-Resolution Transmission Electron Microscopy of Mixed-Layer Illite/Smectite: Computer Simulations
- George D. Guthrie, Jr., David R. Veblen
-
- Journal:
- Clays and Clay Minerals / Volume 37 / Issue 1 / February 1989
- Published online by Cambridge University Press:
- 02 April 2024, pp. 1-11
-
- Article
- Export citation
-
High-resolution transmission electron microscope images of dioctahedral mixed-layer clay structures (illite/smectite) having various substitutional and polytypic schemes were modeled using computer simulation methods. Both one- and two-dimensional calculations were performed using parameters characteristic of a typical range of imaging conditions. One-dimensional images formed by imaging only 00/ diffractions show three important results: (1) The 20-Å periodicity resulting from rigorously ordered R1 illite/smectite can be imaged, but unconventional focus conditions may be necessary. (2) For crystals oriented with the electron beam perfectly parallel to the layers, the brightest fringes in the image correspond to either the octahedral sheets or the interlayer sites, depending on focus conditions. Misorientation of the crystal, however, by only 1° or 2° shifts the positions of the fringes by 1 to 3 Å. Furthermore, in tilted specimens, some defocus values produce images suggesting that smectite layers have a 11–13-Å periodicity, despite the uniform 10-Å periodicity present in the model structure. Thus, direct correlations between image and structure generally should not be made. (3) Two-layer polytypes of pure illite or pure smectite can also produce images with a 20-Å periodicity.
Two-dimensional images additionally showed that the cross fringes produced by some hkl diffractions can be imaged. The simulations showed that these cross fringes ideally might permit the determination of both layer stacking and compositional periodicity, but the fringes are lost by misorientations of a few degrees. These image simulations demonstrated, therefore, that mixed layering of illite and smectite theoretically can be directly imaged by transmission electron microscopy of chemically untreated specimens, but ambiguities may exist in the detailed intepretation of the images.