POLYMORPH AND POLYTYPE IDENTIFICATION FROM INDIVIDUAL MICA PARTICLES USING SELECTED AREA ELECTRON DIFFRACTION
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POLYMORPH AND POLYTYPE IDENTIFICATION FROM INDIVIDUAL MICA PARTICLES USING SELECTED AREA ELECTRON DIFFRACTION ANNE-CLAIRE GAILLOT1 *, VICTOR A. DRITS2, AND BRUNO LANSON3 1
Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky street, 119017 Moscow, Russia 3 Université Grenoble Alpes, CNRS, Université Savoie Mont Blanc, IRD, Université Gustave Eiffel, ISTerre, F-38000 Grenoble, France 2
Abstract—Dioctahedral micas are composed of two tetrahedral sheets and one octahedral sheet to form TOT or 2:1 layers. These minerals are widespread and occur with structures differing by (1) the layer stacking mode (polytypes), (2) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (3) the charge-compensating interlayer cation and isomorphic substitutions. The purpose of the present study was to assess the potential of parallel-illumination electron diffraction (ED) to determine the polytype/ polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns were calculated along several zone axes close to the c*- and c-axes using the kinematical approximation for trans- and cis-vacant varieties of the four common mica polytypes (1M, 2M1, 2M2, and 3T). When properly oriented, all ED patterns have similar geometry, but differ by their intensity distribution over hk reflections of the zero-order Laue zone. Differences are enhanced for ED patterns calculated along the [001] zone axis. Identification criteria were proposed for polytype/polymorph identification, based on the qualitative distribution of bright and weak reflections. A database of ED patterns calculated along other zone axes was provided in case the optimum [001] orientation could not be found. Various polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered. Keywords—Cis-vacant . Dioctahedral mica . Electron diffraction . Illite . Muscovite . Polymorph . Polytype . Trans-vacant . Zone-axis orientation INTRODUCTION K-bearing, aluminous, dioctahedral micas such as illite, muscovite, and phengite, are very abundant in nature and occur, for example, in diagenetically altered sediments, soils, low-grade metamorphic rocks, and igneous and hydrothermal systems (Bailey 1984; Mottana et al. 2004 and references therein). Ideally, their 2:1 layers are composed of a sheet of edge-sharing Al3+ octahedra set between two sheets of Si4+ tetrahedra. In dioctahedral micas, one out of three octahedral sites is vacant. Isomorphic substitutions can occur both in octahedral and tetrahedral sheets to produce a layer-charge deficit which is compensated by interlayer K+ cations. Interactions between adjacent layers are relatively weak and produce energetic similarity of polytypes with different layer stacking. Two or more polytypes can coexist even within a crystal. Whereas polytypes differ mainly by their stacking sequences, polymorphs result from different locations of the octahedral va
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