TWO well known atomic arrangements found among tetrahedrally co-ordinated AX compounds such as silicon carbide (SiC) and zinc sulphide (ZnS) are those based on a cubic close-packed array or on a hexagonal close-packed array of tetrahedra. 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 diamonds2,3. The synthesis of a hexagonal wurtzite polymorph of diamond by shock conversion of graphite has been disclosed in a patent application4. A substance earlier called delta-carbon, synthesized at the General Electric Company, has been characterized by Bundy5 as the wurtzite-like polymorph of carbon under the name hexagonal diamond. While the present work was in progress Hanneman et al.6 put forward X-ray evidence for the occurrence of this polymorph in meteorites.