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

Abstract

The sample investigated was a celadonite specimen named Z1 that has been investigated in several previous works (Drits et al. 1997;Muller et al. 2000aMuller et al. , 2000b. X-ray diffraction indicated that the specimen is completely monomin- LETTER ABSTRACTA new modifi cation 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 = 3a and B = b, where a and b are the cell dimensions of the original mica. Basic h0l refl ections 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 (Fe 3+ ,Mg) cations and one third of the spaces are completely vacant. The (Fe 3+ ,Mg) cations are coordinated by six or fi ve oxygen atoms forming trigonal prisms or square pyramids, respectively.

High-temperature Fe oxidation coupled with redistribution of framework cations in lobanovite, K2Na(Fe2+4Mg2Na)Ti2(Si4O12)2O2(OH)4– the first titanosilicate case

“…650 C based on data obtained by transmission electron microscopy performed with the selected-area electron diffraction technique. The same method allowed for an identification of the new superstructure in annealed celadonite (Kogure et al, 2007). In the HT dehydroxylated modifications of glauconites and celadonites, the ordering of Mg and Fe cations was observed with the formation of a configuration in which each M 3+ cation is surrounded by three M 2+ cations and vice versa (Muller et al, 2000).…”