Abstract
Muscovite (2M1) and biotite (1M) were sheared in situ at room temperature in a 1500-kV transmission electron microscope. The resultant activation of dislocations represents the first success of such an experiment in a rock-forming mineral. Only basal slip was activated in either mica, but the dislocations could be distinguished on micromechanical grounds that may be related to a macroscopic difference in ductility. Invariably, dislocations in the muscovite specimens activated with greater difficulty, despite the greater abundance of precipitates in the biotite specimens. Muscovite basal dislocations were long and roughly linear, oriented along [100] or [110]. They activated in short steps that advanced parallel to the dislocation line, a manner typical of screw dislocations. In contrast, biotite basal dislocations were curvilinear and advanced perpendicular to the dislocation line in bowed segments, characteristic of edge dislocations. The division of dislocation lines, often separated by areas of either reversed contrast or moire fringes, was interpreted as dislocation dissociation. Dissociation appeared to facilitate obstacle circumvention in biotite. The proposed explanation for the micromechanical differences is based on the distribution of Peierls potential energy in the basal glide plane and other energy-minimization arguments. The agreement between the proposed model and observation suggests that the micromechanical difference is structurally determined by the octahedral layer. The proposed Peierls stress control of dislocation activity could be evaluated from natural evidence, based on an expected inverse relationship between the magnitude of the micromechanical differences and temperature.
