The wurtzite ZnS structure type

O Fig- 6.21 The unit cell of rutile (one pol5miorph of Ti02) the titanium centres are shown in grey and the oxygen centres in red. 

The rutile structure type is adopted by Sn02 (cassiterite, the most important tin-bearing mineral), P-Mn02 (pyrolu-site) and Pb02. 

Wurtzite is a second polymorph of ZnS. In contrast to the cubic symmetry of zinc blende, wurtzite has hexagonal symmetry. In the three unit cells shown in Fig. 6.21, the 12 ions in comer sites define a hexagonal prism. Each of the zinc and sulfur centres is tetrahedrally sited, and a unit cell in which Zn and S are interchanged with respect to Fig. 6.21 is equally valid. 

The coordination numbers of titanium and oxygen are 6 (octahedral) and 3 (trigonal planar) respectively, consistent with the 1 2 stoichiometry of mtile. Two of the ions shown in Fig. 6.22 reside fully within the unit cell, while the other four are in face-sharing positions. 

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