Abstract
A dislocation model of the ZnS phase change is presented which accounts for the presence of polytypes, surface striations, banded birefringence, persistent stacking disorder and duplication of unit cells of different polytypes. The possibility that large polytypes result from weak dislocations ensues. Evidence comes from a study of kinks and the determination of the structure, Z(73 24 33)3, of a 66R polytype. The 66R birefringence is proportional to its 2H : 3C ratio.