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Six shale samples, two from core and four from hand-picked cuttings, were examined by transmission electron microscopy (TEM) techniques to study the effects of burial metamorphism on mixed-phase illite/smectite (I/S). TEM lattice fringe images from shallower samples show mixed-phase
I/S layers in subparallel orientations relative to each other, commonly with branching and interfingering relationships. Electron diffraction patterns of these shallower samples show very diffuse basal reflections, prominent turbostratic structure, pronounced streaking along z*, and multiple z* orientations in the same diffraction pattern. All of these features indicate a poorly defined interstratification of the mixed-phase I/S: the diffuse basal reflections are probably due to the presence of only a few layers in a diffraction position; the turbostratic structure is due to misalignment of individual layers relative to each other; the z* streaking is due to stacking disorders of the layers in a direction perpendicular to the layers; and the multiple z SUP>* orientations are due to rotational disorder of the layers about an axis parallel with the layers.
TEM lattice fringe images from deeper samples show mixed-phase I/S layers arranged in a more parallel fashion, with less branching and interfingering. Electron diffraction patterns for these deeper samples show well-defined basal reflections, and both turbostratic structure and z* streaking are less pronounced. These relationships indicate a more regular interstratification for deeper layers: more layers are in diffraction position, the misalignment of individual layers is less evident, and the stacking disorder perpendicular to the layers is less pronounced.
X-ray powder diffractograms have been interpreted to indicate ordering of illite and smectite layers within the mixed-phase I/S. This order is first observed below the "soft" geopressure boundary (0.465 psi/ft) and is prominent below the "hard" geopressure boundary (0.7 psi/ft). However, neither the TEM lattice fringe images nor the electron diffraction patterns show ordering of illite and smectite layers within mixed-phase I/S.
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