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American Mineralogist; May-June; v. 94; no. 5-6; p. 684-692; DOI: 10.2138/am.2009.3059
© 2009 Mineralogical Society of America
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Application of precession electron diffraction to the characterization of (021) twinning in pseudo-hexagonal coesite

Damien Jacob1,*, Patrick Cordier1, Jean-Paul Morniroli2 and Hans-Peter Schertl3

1 Laboratoire de Structure et Propriétés de l’Etat Solide, UMR CNRS 8008, Université des Sciences et Technologies de Lille, Bât. C6, 59655 Villeneuve d’Ascq Cedex, France
2 Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR CNRS 8517, Université des Sciences et Technologies de Lille and Ecole Nationale Supérieure de Chimie de Lille, Bât. C6, 59655 Villeneuve d’Ascq Cedex, France
3 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Correspondence: * E-mail: Damien.Jacob{at}univ-lille1.fr

Precession electron diffraction is used to characterize a (021) twin observed in coesite. Due to the quasi-hexagonal dimensions of coesite (monoclinic space group C12/c1 with β = 120.34°), indexing of conventional spot patterns is ambiguous and the twin law determination is impossible. With precession, spot intensities enable the absolute indexing of the patterns. The method we used is based on the analysis of the departure from hexagonal symmetry. This ensures that all possible pseudo-equivalent monoclinic orientations are taken into account for the indexing. The orientation relationships between adjacent parts of the twin are then characterized. The twin is described as a mirror along (021), which is consistent with original descriptions of twinning in synthetic coesite and a previous characterization performed using large-angle convergent-beam electron diffraction (LACBED).

Key Words: Coesite • twin • TEM • precession electron diffraction






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