Polytypism: A controlled thermodynamic phenomenon

All ZnS polytypes identified so far are listed; their Zhdanov elements show some peculiar statistical characteristics, which are attributed to their formation mechanism. During the cooling down of the crystals hexagonal domains become less and less stable while the stability of cubic domains increases. Transformations take place by the motion of Shockley partials across the basal planes which, due to the existence of screw dislocations, form an interleaved set of helical surfaces. The characteristics of the Zhdanov elements are explained in view of the above mechanism which also accounts for a number of other reported phenomena. Schneer's theory of polytype formation is discussed.