Abstract
We show that hrtem is useful for distinguishing the one-layer polytypes of chlorite and for characterizing layer stacking sequences. hrtem images were computed for all one-layer chlorite polytypes. These computations show that the features in hrtem images can be used to determine the projected shift vectors across the T layers and B sheets. These vectors can, in turn, be used to distinguish among the several possible chlorite polytypes. Five distinct projections result from viewing all possible one-layer chlorite polytypes down their 〈100〉 or 〈110〉 zone axes (assigning [100] parallel to the talc-stagger direction). Complete specification of polytypes requires either hrtem combined with X-ray diffraction analysis or hrtem images from more than one major zone of the same crystal.
A clinochlore sample from West Chester, Pennsylvania, was used as an example of the use of hrtem to study chlorite polytypism. Single-crystal X-ray analysis showed the sample to be an ordered one-layer IIb-2 polytype. Electron diffraction patterns from a much smaller volume of crystal than was used for the X-ray measurements show streaking parallel to c* in the k ≠ 3n reciprocal lattice rows, indicating b/3 shifts of the chlorite layers.
High-resolution imaging of this crystal illustrates the regular alternation of the T layers and B sheets. In addition, semi-random stacking sequences are the dominant type of disorder whereas layer rotations occur less frequently.
