Intergrowth and twinning in opal-CT lepispheres

Abstract

Under conditions of unobstructed diagenetic growth, opal-CT usually forms spherical aggregates, so-called “lepispheres”. From a scanning electron microscope study of deep-sea porcelanites, the development of these structures is described, from isolated opal-CT platelets via complete lepispheres to large coalescent aggregates. The conspicuous symmetric ultrastructure of lepispheres—reflected by the constancy of angles between interpenetrating crystal blades—is crystallographically determined: The lepispheres consist of groups of opal-CT blades intergrown according to the (30–34) and (10–16) twinning laws of tridymite. The same rules of intergrowth can be applied to the morphology of hydrothermally synthesized opal-CT lepispheres.