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Supergene Vermiculitization of A Magnesian Chlorite: Iron and Magnesium Removal Processes

Published online by Cambridge University Press:  02 April 2024

Dominique Proust
Affiliation:
Laboratoire de Pétrologie de la Surface, Université de Poitiers, 86022 Poitiers Cédex, France
Jean-Paul Eymery
Affiliation:
Laboratoire de Métallurgie Physique, Université de Poitiers, 86022 Poitiers Cédex, France
Daniel Beaufort
Affiliation:
Laboratoire de Pétrologie des Altérations Hydrothermales, Université de Poitiers, 86022 Poitiers Cédex, France
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Abstract

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An X-ray powder diffraction study of a vermiculitized chlorite in an amphibole schist near Limoges, France, shows the following weathering sequence: chlorite → ordered interstratified chlorite/vermiculite → vermiculite. Mössbauer spectroscopy indicates that vermiculitization proceeded by the release of ferrous iron from the 2:1 mica layer of the chlorite. The ferric iron content of the vermiculite product is almost the same as that of the initial chlorite. Infrared spectroscopy and chemical microprobe analyses show that Mg was preferentially extracted from the hydroxide sheet of the chlorite, whereas the Si and Al contents progressively increased to the point of the formation of a pure dioctahedral aluminous vermiculite. The Si, Al, and Mg removal processes support currently accepted vermiculitization mechanisms, but the behavior of Fe is slightly different. In this weathering sequence, vermiculitization does not appear to have taken place by the oxidation of Fe2+, but rather, by the simultaneous leaching of Fe2+ and Mg.

Résumé

Résumé

L'analyse par diffraction de rayons X d'une chlorite magnésienne, dans une arène d'amphibolite, caractérise la séquence d'altération suivante: chlorite → interstratifié régulier chlorite/vermiculite → vermiculite. La spectroscopie Mössbauer indique que la vermiculitisation opère par libération du fer, à l’état ferreux, hors du feuillet 2:1 de la chlorite. La teneur en fer ferrique de la vermiculite est voisine de celle de la chlorite. Les analyses à la microsonde et en spectroscopie infrarouge montrent que Mg est préférentiellement libéré de la couche brucitique de la chlorite tandis que les teneurs en Si et Al augmentent avec l'altération jusqu’à la cristallisation d'une vermiculite dioctaédrique alumineuse. Les méchanismes de libération de Si, Al, et Mg sont semblables à ceux habituellement admis pour la vermiculitisation, mais le comportement du fer est différent. Dans cette séquence d'altération, la vermiculitisation ne procède pas par oxydation du fer, mais par départ simultané du fer ferreux et du magnésium.

Type
Research Article
Copyright
Copyright © 1986, The Clay Minerals Society

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