DISLOCATION INTERACTIONS IN EXPERIMENTALLY DEFORMED BIOTITE

Biotite single crystals shortened experimentally in orientations that maximize critical resolved shear stresses on (001) in directions [110], [010] and [310] have been examined by transmission electron microscopy. In all samples, dislocations are confined to the basal plane with Burgers vectors of 1/2[110] and [100], consistent with previous determinations of mica slip systems. Occasional partial dislocations that bound stacking faults form extended dislocations of these same systems. Typically dislocations are concentrated in complex affays in local regions of the samples; isolated or widely spaced dislocations are the exception. Within these arrays, a variety of interactions between the dislocations are suggested by the geometrical arrangements of dislocations that cross over one another on parallel (001) planes sufficiently close together for elastic interaction, and dislocations on the same (001) plane in networks and parallel arrays. Crossing dislocations of opposite sign on separate (001) planes form stable dipole segments where they overlap. Attractive and repulsive interactions between overlapping dislocations of like sign are evident as well. Dislocation networks occur in all samples and may exhibit a variety of complexly distorted geometries. The spacing of dislocations in parallel arrays do not match models for standard dislocation pile-ups, consistent with observations showing thcse arrays involve dislocation multipoles. The observed dislocation arrays and interactions resemble those reported for Stage I easy-glide deformation of FCC and HCP metals. Basal shear strengths are probably largely controlled by the development of multipoles and dislocations that pile up behind them.