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Palaeoweathering in slates from the Iberian Hercynian Massif (Spain): investigation by TEM of clay mineral signatures

Published online by Cambridge University Press:  09 July 2018

M. A. Vicente ,
F. Elsass ,
E. Molina  and
M. Robert
M. A. Vicente
Affiliation:
IRNA/CSIC, Apd 257, 37080 Salamanca, Spain
F. Elsass
Affiliation:
INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
E. Molina
Affiliation:
Dpt Geologia, Universidad Salamanca, Spain
M. Robert
Affiliation:
INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
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Abstract

Palaeoweathering profiles are widely represented in the Iberian meseta. Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) were used to describe the mechanisms governing the processes involved in the formation of one of these weathering profiles developed over slates of the Iberian Hercynian Massif. Three well-differentiated weathering stages were distinguished, based upon well-defined mineralogical signatures characteristic of primary or secondary minerals. The lower stage is characterized by fresh green slates composed of Fe-chlorite, micas, quartz and feldspars. The soft slates represent an intermediate weathering stage. A confined environment has permitted the development of smectite from the different primary phyllosilicates, both by neoformation of montmorillonite and transformation into beidellite. In the upper part of the profile, the red slates are the most weathered. A tropical or subtropical climate has resulted in the formation of kaolinite and abundant iron oxides over several metres. The present study demonstrates that the three identified weathering stages were formed under the same climatic conditions. Such ‘toposequences’ are developed at the present time in tropical areas (Bocquier, 1971).

Type
Research Article
Information
Clay Minerals , Volume 32 , Issue 3 , September 1997 , pp. 435 - 451
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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