In this article, we analyse the common substructure of the Carlsbad twin in terms of the (pseudo)-eigensymmetry of the crystallographic orbits. In the case of the monoclinic polymorphs, all the orbits in the highest subgroup compatible with the twin lattice possess a (pseudo)-eigensymmetry which contains a restoration operation corresponding to the twin operation, which implies a high degree of structural restoration. This structural restoration is realised for twin elements located at distance one half the length of the unit cell, which explains the high occurrence frequency of this twin and its occurrence as both contact and penetration twin, and as primary or synneusis twin. In the case of triclinic polymorphs, the common substructure is much less satisfactory and the Carlsbad twin is probably inherited from the monoclinic phases, accompanied by transformations twins, or by late-stage synneusis, where mechanical adherence on prominent faces plays a more fundamental role.
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JOURNAL ARTICLE
Structural analysis of twins in feldspars. I. Carlsbad twinning
Massimo Nespolo
Massimo Nespolo
1
Université de Lorraine, CNRS, CRM2, UMR 7036
, Vandoeuvre-les-Nancy54506, France
*
Corresponding author, e-mail: massimo.nespolo@univ-lorraine.fr
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Bernd Souvignier
Bernd Souvignier
2
Radboud University Nijmegen, Faculty of Science, Mathematics and Computing Science, Institute for Mathematics, Astrophysics and Particle Physics
, Postbus 9010, 6500 GLNijmegen, The Netherlands
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European Journal of Mineralogy (2017) 29 (6): 939-947.
Published:
November 27, 2017
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Massimo Nespolo, Bernd Souvignier; Structural analysis of twins in feldspars. I. Carlsbad twinning. European Journal of Mineralogy ; 29 (6): 939–947. doi: https://doi.org/10.1127/ejm/2017/0029-2678
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