Fine structure in the selected area diffraction patterns of beidellite, and its dark-field images

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Abstract
Commonly streaks and rarely extra spots are observed on the selected area diffraction patterns of beidellite. The streaks are non-radial, and are restricted along the direction parallel to [11], [11], and [01]. Extra spots appear instead of streaks between the normal Bragg spots along the direction parallel to [11] and [11]. These extra spots suggest the formation of a superlattice with the a and b cell parameters twice as large as those of the beidellite lattice. Dark-field images on the strong 020 reflection reveal the presence of a network of dislocations in some of the beidellite crystallites. These dislocations suggest that the chemistry is not uniform over the successive layers in these crystallites. The structural difference between the layers seems to be accommodated by the dislocations.