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Modulated structures in calcian dolomite: A study by electron microscopy |
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| Authors: | G Van Tendeloo H R Wenk R Gronsky |
| Institution: | 1. Laboratory for Solid State Physics, University of Antwerp (RUCA), Groenenborgerlaan 171, 2020, Antwerp, Belgium 2. Department of Geology and Geophysics, University of California, 94720, Berkeley, CA, USA 3. National Center for Electron Microscopy, Lawrence Berkeley Laboratory, University of California, 94720, Berkeley, CA, USA |
| Abstract: | Calcian dolomite from the Devonian Lost Burro formation has been investigated with electron microscopy techniques. Electron diffraction shows evidence for “c” and “d” type reflections which may occur independently and are indicative of ordered superstructures. High resolution electron microscopy combined with selected area optical diffraction is the basis for models to explain the superstructures in calcian dolomite. It is proposed that “c” reflections are due to ordered substitution of Mg by Ca in basal cation layers. “d” reflections result when the rhombohedral stacking of basal layers is interrupted by intercalation of additional Ca layers. During electron irradiation at 1 MeV the Mg-Ca distribution becomes disordered and the crystal structure attains calcite symmetry. The arrangement of CO3 groups remains ordered. |
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