Slip and dislocation behaviour in dolomite

The behaviour of dislocations generated in dolomite single crystals, experimentally 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 dissociation occurs, i.e. basal, c = (0001) and rhombohedral f = ({(1) over bar 012} 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 system is impeded by debris produced by dislocation interactions and by a tendency for slip to disturb the regular arrangement of CO3 groups, with such disordering being favoured at elevated temperatures. It is shown that the repeated motion of dissociated dislocations on the f-planes creates thin blocks of a second phase, structurally related to dolomite, but with double the a-lattice parameter. There is little experimental evidence for the occurrence of complex dislocation dissociations which might be expected. Obstacles to slip on the f- and r- planes are generally overcome by the formation of dipoles and their pinching off, with double cross-slip as a competing mechanism. Corresponding mechanisms are largely lacking or inoperative during basal slip. These differences appear to explain the anomalous increase in yield stress and strength with increasing temperature observed by several investigators, which is manifest when single crystals deform predominantly by basal slip.