Twin structure in coesite studied by high resolution electron microscopy

A synthetic crystal of coesite, SiO₂, has been found to contain microtwins. The (100) reflection twin has been made evident on incoherent (010) planes. The twin structure is directly imaged by high resolution electron microscopy at 2,5 Å resolution, allowing an atomic model to be proposed. This twin could be incoherent on large surfaces and the dislocation density is predicted. The X-ray crystallographic data are analyzed critically in view of the presence of such defects in single crystals. The mutual orientation between a silicon matrix and coesite precipitates in Czochralski-silicon annealed specimens is proposed to be non unique as a consequence of the easy twinning of coesite.