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Mica weathering in acidic soils by analytical electron microscopy

Published online by Cambridge University Press:  09 July 2018

H. Aoudjit ,
F. Elsass ,
D. Righi  and
M. Robert
H. Aoudjit
Affiliation:
Unité de Science du Sol, INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
F. Elsass
Affiliation:
Unité de Science du Sol, INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
D. Righi
Affiliation:
URA 721, CNRS, “Argiles, Sols et Altérations”, Faculté des Sciences, 86022 Poitiers Cedex, France
M. Robert
Affiliation:
Unité de Science du Sol, INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
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Abstract

The mineralogy, crystallochemistry and microfabric of clay minerals from acidic soils were studied using transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Soil profiles, developed on saprolites, sampled in the main crystalline massifs of France represent different pedological environments. The study focused on the microsystem of mica weathering, which appeared to be the main source of secondary clay minerals, and involves microdivision, transformation and dissolution. Microdivision begins with the splitting of large particles along layer planes and their shearing normal to the layers. This induces the breakdown of particles of one hundred layers into particles having only a few layers. The transformation of micas follows two steps: they first transform into 1–1.4 nm mixed-layer minerals and then into hydroxy-Al interlayered vermiculite. The formation of hydroxy-Al interlayered vermiculite derived from micas is dominant in acidic soils; particles are generally small, consisting of only three to seven layers, and always have a dioctahedral composition, whatever the type of the original mica (trioctahedral or dioctahedral). Dissolution affects the surface layers or large domains of the core of the particles and leads to the formation of multi-elementary gels rich in Fe and Al.

Resume

Resume

La minéralogie, la cristallochimie et la microorganisation de minéraux argileux de sols acides ont été étudiées par microscopie électronique à transmission (TEM) et microanalyse X (AEM). Des profils de sols ont été prélevés dans les principaux massifs cristallins de France. Ces sols développés sur arènes repréntent divers environnements pédologiques. L'étude est focalisée sur le microsystème d'altération des micas qui apparaissent être la principale source de minéraux argileux secondaires. L'altération des micas se fait par microdivision, transformation et dissolution. La microdivision commence par le délitement des grandes particules suivant le plan des feuillets et leur cisaillement perpendiculairement aux feuillets. Ceci entraine l'effritement des particules ayant une centaine de feuillets en particules plus petites n'ayant que quelques feuillets. La transformation des micas comprend deux étapes: la première est représentée par des minéraux interstratifiés 1–1.4 nm, la deuxième par la vermiculite hydroxyalumineuse. La transformation en vermiculite hydroxyalumineuse est un phénomène majeur dans les sols acides; les particules sont en général de petite taille (3–7 feuillets), et ont toujours une composition dioctaédrique, quelque soit le mica d'origine (trioctaédrique ou dioctaédrique). La dissolution affecte les feuillets de surface ou bien des domaines étendus au coeur des particules. Elle entraine la formation de gels multi-élémentaires riches en Fe et Al.

Type
Research Article
Information
Clay Minerals , Volume 31 , Issue 3 , September 1996 , pp. 319 - 332
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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