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Infrared study of reduced and reduced-reoxidized ferruginous smectite

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Oxidation-reduction processes within natural systems greatly influence the properties of sediments, soils and clays. The objective of this experimental study was to gather new evidence for the effects of changes in redox conditions (reduction and reoxidation) on structural properties of ferruginous smectite and to understand better the mechanisms involved. The <2 ┬Ám fraction of a ferruginous smectite (sample SWa-1), which contains 17.3 wt.% of total structural Fe, was studied by infrared (IR) spectroscopy. The pure Na-saturated clay was reduced by Na dithionite for 10 to 240 min to obtain various Fe(II):(total Fe) ratios ranging from 0 to 1.0. Selected reduced samples were then reoxidized completely by bubbling O2 gas through the suspensions for up to 12 h. Infrared spectra of the initially unaltered, reduced and reduced-reoxidized samples were collected. Reduction generated changes in the three studied spectral regions (O-H stretching, M-O-H deformation, and Si-O stretching), indicating that major modifications occurred within the clay crystal beyond merely a change in Fe oxidation state. Partial dehydroxylation and redistribution of Fe, and perhaps Al, cations occurred upon reduction of SWa-1, changing the structural properties of its tetrahedral and octahedral sheets. Water molecules, probably generated by dehydroxylation within the octahedral sheet upon reduction, were tightly bound to the clay surface and were possibly trapped within the clay structure. Except for dehydroxylation and the Fe oxidation state, all these modifications were largely irreversible. The tightly bound water was not completely removed upon reoxidation and the cationic rearrangements generated during reduction were not reversed: either they were preserved as in the reduced state or cations were redistributed into a different configuration from the unreduced clay.
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Keywords: CATIONIC REARRANGEMENT; INFRARED SPECTROSCOPY; IRON; IRREVERSIBILITY; OH DEFORMATION; OH STRETCHING; REDUCTION; REOXIDATION; SI-O STRETCHING; SWA-1 SMECTITE

Document Type: Research Article

Publication date: 01 August 2002

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

    Clays and Clay Minerals is the official publication of The Clay Minerals Society.

    The Editor-in-Chief is Professor Joseph W. Stucki [email protected]

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