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Pedogenic formation of smectites in a vertisol developed from granitic rock from Kaélé (Cameroon, Central Africa)

Published online by Cambridge University Press:  09 July 2018

J. P. Nguetnkam
Affiliation:
Département des Sciences de la Terre, Faculté des Sciences, Université de Ngaoundéré, BP 454, Ngaoundéré, Cameroon Laboratoire Environnement et Minéralurgie, UMR 7569, Nancy-Université - CNRS, BP 40, 545001 Vandoeuvre-lès-Nancy, France
R. Kamga
Affiliation:
Département de Chimie Appliquée, Ecole Nationale Supérieure des Sciences agroindustrielles, Université de Ngaoundéré, BP 455, Ngaoundéré, Cameroon
F. Villiéras*
Affiliation:
Laboratoire Environnement et Minéralurgie, UMR 7569, Nancy-Université - CNRS, BP 40, 545001 Vandoeuvre-lès-Nancy, France
G. E. Ekodeck
Affiliation:
Département des Sciences de la Terre, Faculté des Sciences, Université de Yaoundé I, BP 812, Yaoundé, Cameroon
J. Yvon
Affiliation:
Laboratoire Environnement et Minéralurgie, UMR 7569, Nancy-Université - CNRS, BP 40, 545001 Vandoeuvre-lès-Nancy, France
*

Abstract

Smectite formation in a vertisol developed from a granitic parent rock in the Kaélé region of Cameroon in a tropical, dry climate was studied by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), chemical analyses, cation exchange capacity (CEC) and low temperature gas adsorption. The soil profile comprises three horizons (from base to top): (1) a saprolite, (2) an intermediate horizon, and (3) an upper dark grey horizon. In the saprolite, the progressive alteration of feldspars gave rise to the exclusive neoformation of beidellite exhibiting a typical honeycomb fabric. This process resulted in Al, Fe, Ca, Mg, Ti and Mn enrichment, a depletion of Si and Na and a significant negative Eu anomaly. In the upper horizons, beidellite evolves and is transformed into montmorillonite and kaolinite, probably through a series of smectite-kaolinite mixed layers. This transformation, from the saprolite to the upper horizons, causes the observed decrease in the CEC and the increase in specific surface area and mesoporosity of the clay. The REE patterns of the bulk soil and clay fraction display similar behaviour, indicating that the three horizons, and hence the clay minerals, have a common source in the underlying granite. Mass-balance calculations show that the intense weathering of the granite leads to a mass reduction of ~80–90%.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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