POLYMORPH AND POLYTYPE IDENTIFICATION FROM INDIVIDUAL MICA PARTICLES USING SELECTED AREA ELECTRON DIFFRACTION

International audienceDioctahedral 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 (i) the layer stacking mode (polytypes), (ii) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (iii) the charge-compensating interlayer cation and isomorphic substitutions. The potential of parallel-illumination electron diffraction (ED) is assessed here to determine the polytype/polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns are 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 (1 M , 2 M 1 , 2 M 2 , and 3 T). 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 are 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 is provided in case the optimum [001] orientation cannot be found. Different polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered