Question

Although silicon and carbon are in the same group of the periodic table and do not share the diamond cubic crystal structure (for one polymorph of carbon) but look very different when in the diamond cubic crystal structure compare how the two polymorphs with the diamond cubic crystal structure look (ie compare how silicon looks to how diamond looks)

Answer 1

The two polymorphs are generally designated as the sphalerite and wurtzite types, respectively, from their occurrence in zinc sulphide. The crystal structures of the Group IV elements, carbon, silicon, germanium and α-tin, are based on the cubic close-packed sphalerite arrangement (aside from graphite, tetragonal tin and tetragonal germanium) with all A and X sites occupied by the same kind of atom. The existence of polymorphs analogous to wurtzite among these elements is not surprising, because the two types of structure are so similar geometrically that the energy differences between them must be small. A wurtzite-like polymorph of silicon has already been synthesized¹. For carbon a rhombohedral polymorph called beta-diamond, which corresponds to the 3R polytype of wurtzite, has been reported as birefringent lamellae in terrestrial diamonds.