Influence of stacking faults on the spiral growth of polytype structures in mica

Abstract

A systematic theoretical deduction of polytype structures of mica that can result by the spiral growth mechanism operating in faulted 1M, 2M₁ and 3T basic matrices is reported. As a prerequisite, all possible intrinsic and extrinsic stacking fault configurations in each of the basic matrices have been worked out and their stacking fault energy (SFE) estimated. The deduction of polytype structures on the basis of the "faulted-matrix model" takes into account (i) the introduction of each of the low energy fault configurations in the exposed ledge of the screw dislocations, (ii) the change in the layer-position of the fault within the exposed ledge and (iii) the variation of the strength of the generating screw dislocation. At each stage, the spirally-grown polytypes are deduced for each basic structure. The most probable structures are predicted on the basis of the lowest SFE for the same strength of the screw dislocation and are then compared with the polytype structures reported in the literature. It was found that the faulted matrix model accounts successfully for the origin of all the polytype structures in mica. Furthermore, it may provide a basis for limiting the number of trial structures for determining the structures of long period polytypes.