'Black' soils in the southern Alps : Clay mineral formation and transformation, x-ray amorphous Al phases and Fe forms
Zanelli, R. (University of Zurich. Department of Geography)
Egli, M. (University of Zurich. Department of Geography)
Mirabella, A. (Research Institute for Soil Study and Conservation)
Abdelmoula, Mustapha. (Laboratoire de Chimie Physique et Microbiologique pour l'Environnement)
Plötze, Michael (Institute for Geotechnical Engineering.)
Nötzli, M. (University of Zurich. Department of Geography)

Date:	2006
Abstract:	Many soils in southern Switzerland have a black color, contain a large amount of soil organic matter (SOM) and seem to have some andic properties although they did not develop on volcanic parent material. We investigated three typical 'black' soils to determine the mechanisms of (clay) mineral formation and transformation. We measured total element pools as well as the dithionite-, pyrophosphate-and oxalate-extractable fractions (Fe, Al, Si). The clay fraction (<2 μm) was analyzed using X-ray diffraction and FTIR spectroscopy. Iron speciation in the solid phase was determined by Mössbauer spectroscopy. With increasing weathering conditions, the plagioclase (albite) content decreases, trioctahedral species in the clay fraction such as biotite, chlorite or trioctahedral vermiculite either decompose or transform into a dioctahedral mineral such as dioctahedral vermiculite or hydroxy interlayered smectite (HIS). Typical weathering products were hydroxy interlayered vermiculite (HIV), HIS, interstratified minerals and kaolinite. The oxidation of Fe(II) into Fe(III) was evident and contributes to the transformation of trioctahedral mineral species into dioctahedral ones. In one soil, a large part of the Fe (up to 41%) was found in the form of Fe oxides. In the surface horizon, the poorly crystalline mineral ferrihydrite was dominant, while in the subsoil goethite prevailed. Maghemite (or maghemite/hematite mixture) was, furthermore, found in distinct concentrations down to a depth of ∼50 cm. The formation of this mineral requires high temperatures which means that a forest fire can influence soil mineralogy down to a considerable depth. The specific climatic conditions with periods of strong humidity alternating with periods of winter droughts, sporadic fire events and the relatively large content of poorly crystalline fractions of Fe and Al contributed to the stabilization of SOM. Copyright © 2006, The Clay Minerals Society.
Rights:	Tots els drets reservats.
Language:	Anglès
Document:	Article ; recerca ; Versió publicada
Subject:	Fe-speciation ; Fire ; Maghemite ; Soil Minerals ; Weathering
Published in:	Clays and Clay Minerals, Vol. 54, Núm. 6 (2006) , p. 703-720, ISSN 1552-8367

DOI: 10.1346/CCMN.2006.0540606

18 p, 1.1 MB  UAB restricted access

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