Novel 2:1 structure of phyllosilicates formed by annealing Fe3+, Mg-rich dioctahedral mica

Abstract

A new modification of the 2:1 phyllosilicate layer has been discovered in annealed celadonite, a Fe 3+ , Mg-rich dioctahedral mica. Plan-view diffraction patterns in TEM indicate a base-centered supercell with A = 3 a and B = b , where a and b are the cell dimensions of the original mica. Basic h 0 l reflections with h = 3 n form an orthogonal lattice with one-layer periodicity, which is not expected for normal micas. The high-resolution TEM image along the or related directions is similar to that expected from normal micas but the image along the 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 (Fe 3+ ,Mg) cations and one third of the spaces are completely vacant. The (Fe 3+ ,Mg) cations are coordinated by six or five oxygen atoms forming trigonal prisms or square pyramids, respectively.