The differences in stacking discussed in Sections 1.2 and 1.6 also explain the different crystal structures or different crystalline polymorphs sometimes shown by one compound. As mentioned above, zinc blende has an fcc structure, the sulphur atoms being stacked in the ABCABC.. sequence. In wurtzite the other crystal structure or polymorph of zinc sulphide, the sulphur atoms are stacked in the hexagonal ABABAB... sequence, giving a hexagonal structure. In both cases the zinc atoms occupy half the tetrahedral interstitial sites between the sulphur atoms. As in the case of cobalt, stacking faults may arise during crystal growth or under conditions of deformation, giving rise to 'mixed' sphalerite-wurtzite structures.