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Novel 2:1 structure of phyllosilicates formed by annealing Fe³⁺, 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, Moscow, Russia

Correspondence: ^* E-mail: kogure{at}eps.s.u-tokyo.ac.jp

A new modification of the 2:1 phyllosilicate layer has been discovered in annealed celadonite, a Fe³⁺, Mg-rich dioctahedral mica. Plan-view diffraction patterns in TEM indicate a base-centered supercell with A = 3a and B = b, where a and b are the cell dimensions of the original mica. Basic h0l reflections with h = 3n form an orthogonal lattice with one-layer periodicity, which is not expected for normal micas. The high-resolution TEM image along the <100> or related directions is similar to that expected from normal micas but the image along the <010> directions is completely different. From these images, it is concluded that the two tetrahedral sheets in a 2:1 layer are facing each other with no lateral a/3 stagger. In the proposed model that explains the high-resolution TEM images, two thirds of the spaces surrounded by two facing tetrahedral six-member rings accommodate three (Fe³⁺,Mg) cations and one third of the spaces are completely vacant. The (Fe³⁺,Mg) cations are coordinated by six or five oxygen atoms forming trigonal prisms or square pyramids, respectively.

Key Words: Crystal structure • electron microscopy • electron diffraction • mica • phase transition • phyllosilicates

This article has been cited by other articles:

				T. Kogure, J. Kameda, and V. A. Drits STACKING FAULTS WITH 180{degrees} LAYER ROTATION IN CELADONITE, AN Fe- AND Mg-RICH DIOCTAHEDRAL MICA Clays and Clay Minerals, December 1, 2008; 56(6): 612 - 621. [Abstract] [Full Text] [PDF]