ZnS (zinc blende): $S^{2-}$ in FCC, $Zn^{2+}$ in alternate tetrahedral voids (half of 8 = 4). Z = 4, CN 4:4. Face diagonal contains the relationship: a*sqrt$\frac{3}{4}$ = r+ + r-. Examples: ZnS, CuCl, SiC. CaF2 (fluorite): $Ca^{2+}$ FCC, F- in ALL tetrahedral voids (8). Z = 4 CaF2, CN $Ca^{2+}$ = 8, F- = 4. Antifluorite (Na2O): $O^{2-}$ FCC, Na+ in all tetrahedral voids.