Wurtzite, a Polymorph of Sphalerite

How many layers of closest-packed sulfide ions are shown  

Notice that the layers alternate in the hexagonal closest-packed sequence ABAB. .. The zinc ions occupy, as they did in sphalerite, one-half of the tetrahedral voids. 

This set of unit cells shows that there is no six-fold rotational axis along the c-axis (remember that the top view looks down the c-axis). However, there is a three-fold rotational axis, and this is sufQcient to place wurtzite in the hexagonal system. 

---