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Using [delta] ph as a geochemical index of illite neoformation in saprolite

Bétard François, Caner Laurent, Gunnell Yanni, Bourgeon Gérard. 2008. Using [delta] ph as a geochemical index of illite neoformation in saprolite. In : EUROSOIL 2008 Soil - Society - Environment, Vienne, 25-29 août 2008. ECSSS. Vienne : ECSSS, 1p. Eurosoil 2008, Vienne, Autriche, 25 Août 2008/29 Août 2008.

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Résumé : Sal pH is routinely measured for agronomic purposes. When the difference between KCI pH and H2O pH, or ApH, yields positive values, it is used by soil scientists as a classification criterion for identifying anionic subgroups according to the Soil Taxonomy or geric properties according to the WRB. Negative values have not been granted much attention. Here we focus on the occurrence of highly negative ApH values in the weathering zone of profiles developed on gneiss in semiarid Northeast Brazil and semiarid South India and interpret them as proxies of a geochemical weathering process involving the neoformation of illite. Detailed optical, chemical and mineralogical characterizations involving scanning electron microscopy coupled with X-ray element mapping demonstrate the neoformation of illite inside plagioclase feldspar crystals after their partial dissolution. This study thus reveals that meteoric weathering is capable of producing Mite not only from mica, Le., by a transformation process, but also within non-alkali feldspar by a neoformation process. The ApH is shown to be a good proxy for detecting such weathering signatures because the recently neoformed Mate flakes, which present a significant compositional deficit in K, reveal their presence by a detectable uptake of K from the KCI solution. This finding changes the perspective over the origin of illite in continental environments, which has most commonly been attributed to hydrothermal processes. (Texte intégral)

Mots-clés Agrovoc : sciences du sol

Mots-clés géographiques Agrovoc : Brésil, Inde

Classification Agris : P33 - Chimie et physique du sol
U10 - Informatique, mathématiques et statistiques

Auteurs et affiliations

  • Bétard François, Université de Paris-Val-de-Marne (FRA)
  • Caner Laurent, Université de Poitiers (FRA)
  • Gunnell Yanni, CNRS (FRA)
  • Bourgeon Gérard, CIRAD-PERSYST-UPR Recyclage et risque (FRA)

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Source : Cirad - Agritrop (https://agritrop.cirad.fr/546229/)

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