Cation distribution, sheet thickness, and O-OH space in trioctahedral chlorites - an X-ray and infrared study.

Abstract

One-dimensional refinement and IR OH band investigations have been made on five orthochlorites of the Mg-Fe series and on two Mg-leptochlorites with substitution of 2Al for 3Mg; results indicate that the leptochlorites have cation vacancies in the interlayer sheet. Octahedral Al is concentrated in the interlayer sheet in every sample, but considerable amounts of Al are contained also in the 2:1 octahedral sheet in the Al- and/or Fe-rich chlorites, which suggests a relatively narrow layer charge range in chlorites. Changes of cation composition in the tetrahedral and octahedral sheets result in variation of the sheet thickness and the interlayer O-OH space: with increasing tetrahedral Al (usually octahedral Al also increases), the two octahedral sheet thicknesses and the O-OH space decrease, but the tetrahedral sheet thickness increases; larger Fe2+ content brings about thicker octahedral and thinner tetrahedral sheets and smaller O-OH space. The O-OH space, which primarily determines the d001 spacing, is controlled by both the surplus negative charge of 2:1 layer surface oxygens and the interlayer octahedral cations; the former is caused by the substitution of tetrahedral Al for Si and is also affected by the 2:1 octahedral cations. The OH bands and tetrahedral ordering of amesite and the O-OH spaces of dioctahedral chlorites and of some 1:1 layer silicates are discussed along with those of trioctahedral chlorites.-T.Y.