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POLYTYPE AND MORPHOLOGICAL ANALYSES OF GÜMBELITE, A FIBROUS Mg-RICH ILLITE

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 Department of Geology, The National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan
³ Geological Institute, Russian Academy of Science, Pyzhevsky per. D7, 1109017 Moscow, Russia

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

Polytypism in gümbelite and its relationship to the fibrous or ribbon-like morphology exhibited by this Mg-rich illite were investigated by powder X-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Comparison between the XRD pattern from oriented fibers using a conventional powder diffractometer and a randomly oriented pattern using a Gandolfi camera suggested that 2M₂ is dominant but other polytypes belonging to subfamily A also exist, and that the fiber axis of gümbelite is parallel to <110> in 2M₂, <110> in 2M₁, and <100> in 1M. The EBSD analyses confirmed these crystallographic directions directly from individual crystals. Electron diffraction and high-resolution TEM showed that twinning and intergrowths of various polytypes including both subfamilies are common in a single crystal and that the two types of rotations [2n60° and (2n+1)60°] between adjacent layers are often randomly mixed at the monolayer level. The data suggest that high densities of twinning and intergrowths account for the origin of the fibrous morphology along <110> for 2M₁ and 2M₂ polytypes. Volume restriction in a confined vein space may also play a role.

Key Words: EBSD • HRTEM • Illite • Morphology • Polytype • Stacking Disorder

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