Wurtzite, limiting structure compounds

Among the covalent AX compounds which crystallize with either the zincblende or wurtzite structure are ZnS itself, ZnO, AIN, A1P, HgS, CuCl and many more. At first sight it would seem that such an arrangement is inconsistent with the known covalencies of the atoms involved. In ZnS, for example, the zinc and sulphur atoms, with two and six electrons, respectively, in the outermost shell, would be expected to be limited to a covalency of 2 and therefore to be capable of covalent... 

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