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Contrasted mineralogical composition of the laterite cover on serpentinites of Nkamouna-Kongo, southeast Cameroon

¹ Department of Earth Sciences, Faculty of Science, University of Yaoundé I, PO Box 812 Yaoundé, Cameroon, ² Institute of Agricultural Research for Development (IRAD), PO Box 2067 Yaoundé, Cameroon, ³ Department of Earth Sciences, Faculty of Science, University of Dschang, PO Box 67, Dschang, Cameroon, ⁴ Department of Earth Sciences, Faculty of Science, University of Douala, PO Box 24157 Douala, Cameroon

^* E-mail: rfyongue{at}yahoo.fr

(Received 11 November 2007; revised 17 December 2008)

Four drill cores along a lateritic hill in Nkamouna-Kongo (southeast Cameroon) were studied using microprobe analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and infrared spectroscopy. The main objective of the study was to investigate the occurrence of clay silicates while evaluating the Ni-Co content of the laterite for potential mineral exploitation. The thick lateritic profiles (>40 m deep) developed on serpentinized ultramafic rocks have an Fe-rich clayey fraction, with goethite as the main mineralogical constituent and secondary quartz and relicts of magnetite-maghemite as accessory minerals. Silicate clays are less abundant and occur mainly towards the top of the profiles. At the summit of the interfluve, kaolinite and some gibbsite are associated with goethite. Along the slopes, at the bottom of profiles, the weathered bedrock shows the presence of smectite (Fe-beidellite) and kaolinite, probably due to slower water-flow conditions. Towards the top of the profile in the lower clay, ferruginous and upper clay horizons, only kaolinite remains with gibbsite, after the leaching of silica and soluble cations following repeated remobilization-recrystallization processes. At the summit, where the downward movement of water is rapid, no smectite was identified. As a whole, the mineralogical composition of the material varies in close relationship with the drainage, leading to a contrasted clay mineralogy marked by the presence of Fe-beidellite and kaolinite at the bottom, and that of kaolinite and gibbsite at the summit of the profiles.

KEYWORDS: lateritic cover, serpentinite, goethite, gibbsite, kaolinite, smectite, Cameroon