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POLYTYPE AND MORPHOLOGY ANALYSES OF KAOLIN MINERALS BY ELECTRON BACK-SCATTERED DIFFRACTION

¹ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
² CREST, Japan Science and Technology Corporation, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan
³ Department of Earth Sciences, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
⁴ HydrASA UMR 6532 CNRS, Université de Poitiers, 40 Av. Recteur Pineau, 86022 Poitiers Cedex, France

^* E-mail address of corresponding author: kogure{at}eps.s.u-tokyo.ac.jp

The electron back-scattered diffraction (EBSD) technique has been applied to fine-grained kaolin minerals to determine the polytypes (kaolinite, dickite and nacrite) of individual grains and their crystallographic orientations in a scanning electron microscope (SEM). Because kaolin minerals are prone to radiation damage by the intense electron beam necessary to obtain EBSD patterns, the beam has to be rastered across the specimens during pattern acquisition. Kaolinite-dickite and nacrite are easily distinguished by their trigonal or hexagonal symmetry about the [001]* direction, respectively, of the Kikuchi bands with k = 3n. Dickite can be differentiated from kaolinite by mirror symmetry parallel to the a–c plane, and by the characteristic contrast of HOLZ rings.

As EBSD analysis is performed using a SEM, morphological characters can be correlated with polytypes. Dickite fragments could be discerned as oriented overgrowths on nacrite plates. Dickite was observed to always adopt euhedral shapes in a diagenetic mixture of kaolinite and dickite. Lath-shaped dickite is elongated along the a axis direction. Rhombohedral dickite consists of crystals with {11} indexed to the side facets.

Key Words: Dickite • EBSD • Kaolin Minerals • Kaolinite • Morphology • Nacrite • Polytype • SEM

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