Slip and dislocation behaviour in dolomite

The behaviour of dislocations generated in dolomite single crystals, experi mentally deformed at 300- 420 degreesC, is analysed by TEM with emphasis on the effects of dislocation dissociation. It is shown that the behaviour of dislocations is very different on the two major slip systems where dissoci ation occurs, i.e. basal, c = (0001) and rhombohedral f = ({(1) over bar 01 2} slip. Dissociation on the minor r = {10 (1) over bar4} slip planes seems to be impossible. The glide of dislocations on all three types of slip sys tem is impeded by debris produced by dislocation interactions and by a tend ency for slip to disturb the regular arrangement of CO3 groups, with such d isordering being favoured at elevated temperatures. It is shown that the re peated motion of dissociated dislocations on the f-planes creates thin bloc ks of a second phase, structurally related to dolomite, but with double the a-lattice parameter. There is little experimental evidence for the occurre nce of complex dislocation dissociations which might be expected. Obstacles to slip on the f- and r- planes are generally overcome by the for mation of dipoles and their pinching off, with double cross-slip as a compe ting mechanism. Corresponding mechanisms are largely lacking or inoperative during basal slip. These differences appear to explain the anomalous incre ase in yield stress and strength with increasing temperature observed by se veral investigators, which is manifest when single crystals deform predomin antly by basal slip.