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STACKING FAULTS WITH 180° LAYER ROTATION IN CELADONITE, AN Fe- AND Mg-RICH DIOCTAHEDRAL MICA

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 Geological Institute of the Russian Academy of Sciences, Pyzhevsky per 7, 119017 Moscow, Russia

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

Stacking disorder in celadonite, a dioctahedral mica with Fe and Mg as major octahedral cations and which generally adopts the 1M stacking sequence, was investigated mainly by using transmission electron microscopy (TEM). The selected-area electron diffraction patterns with 0kl reflections along the [100] beam direction correspond to the 1M stacking but those along the [110], [10], [010], [310], and [30] directions are frequently streaked along the [001]* direction or contain extra spots from twinned domains. Three-dimensional stacking analyses using sets of two high-resolution TEM images along different directions of the same area of one crystal indicate that all stacking faults involve only 180° layer rotations. These stacking faults produce greater peaks of 0kl reflections than expected in powder X-ray diffraction (XRD) patterns. Simulation of the XRD patterns indicated that 180° layer rotations occur at >10% of total interlayer regions in one celadonite specimen. The interlayer region of celadonite is characterized by a near-zero ditrigonal rotation angle, a small surface corrugation of the basal oxygen plane, and a small amount of Al substitution in the tetrahedral sheets. These features suggest that there is no preference for any of the six stacking angles around the interlayer region. The abundance of 180° layer rotation rather than ±60° and ±120° in the present specimens may be related to their ribbon-like morphologies elongated along the a axis.

Key Words: Celadonite • Electron Microscopy • Mica • Polytype • Stacking Faults • X-ray Diffraction