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 microsco py. In all samples, dislocations are confined to the basal plane with Burgers vectors of 1/2[110] and [100], consistent with previous determ inations of mica slip systems. Occasional partial dislocations that bo und stacking faults form extended dislocations of these same systems. Typically dislocations are concentrated in complex affays in local reg ions of the samples; isolated or widely spaced dislocations are the ex ception. Within these arrays, a variety of interactions between the di slocations are suggested by the geometrical arrangements of dislocatio ns that cross over one another on parallel (001) planes sufficiently c lose together for elastic interaction, and dislocations on the same (0 01) plane in networks and parallel arrays. Crossing dislocations of op posite sign on separate (001) planes form stable dipole segments where they overlap. Attractive and repulsive interactions between overlappi ng 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 mat ch models for standard dislocation pile-ups, consistent with observati ons 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 d islocations that pile up behind them.