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

Dominique Proust1, Jean-Paul Eymery2 and Daniel Beaufort3
1 Laboratoire de Pétrologie de la Surface, Université de Poitiers 86022 Poitiers Cédex, France
2 Laboratoire de Métallurgie Physique, Université de Poitiers 86022 Poitiers Cédex, France
3 Laboratoire de Pétrologie des Altérations Hydrothermales, Université de Poitiers 86022 Poitiers Cédex, France

Abstract: 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.

Key Words: Chlorite • Infrared spectroscopy • Interstratified chlorite/vermiculite • Iron • Mössbauer spectroscopy • Vermiculite • Weathering

Clays and Clay Minerals; October 1986 v. 34; no. 5; p. 572-580; DOI: 10.1346/CCMN.1986.0340511
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)