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a b s t r a c tThe physicochemical, mineralogical and technological properties of clayey materials from the Ndop plain (NW Cameroon) have been investigated, for their ceramic applications. The clayey materials have mixed facies with colour ranging from brown, grey, mottled and yellowish brown. The thickness of the exploitable clay layer is more than 5 m. Their mineralogical constituents are quartz, kaolinite, illite and feldspars, with kaolinite as major clay mineral. Based on the geochemistry, their source rocks might be felsic, with a mafic rock inference. These materials display high percentage in fine particles and high Atterberg limits. For all the firing temperatures, flexural strength (1.2-11 MPa), water absorption (8.03-24.27%), linear shrinkage (2.10%), weight loss (4.88-16.54%) and bulk density (1.57-2.03 g/cm 3 ) indicate good ceramic properties for firing samples between 900 and 1100 • C. Most of the fired test bricks show a brick red colour with good to very good cohesion. The studied clays were thus, suitable as raw materials for roof tiles, light weight blocks and hollow bricks; however, mixing of high clayey and highly silty materials could improve the quality of the products.

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Editor: Frédéric Villiéras)AB ST R ACT : 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. Southern Cameroon is located in the tropical rain forest zone, where the climate is hot and humid (annual precipitation of 1500 mm and average temperature of 23ºC; Suchel, 1987). Weathering of the rocks there essentially produces a lateritic cover. The profiles display various morphologies and thicknesses. The lateritic cover consists mainly of kaolinite and gibbsite as the result of monosiallitization and allitization (formation of 1:1 clays and of Al hydroxides, respectively) processes (Pedro, 1987).In tropical and subtropical climates, prolonged weathering of ultramafic rocks usually leads to the formation of nickel laterite deposits, which can be classified either as hydrous silicate deposits, clay silicate deposits or oxide deposits, on the basis of the mineralogy of principal ore-bearing phases (Brand et al., 1998, in Gleeson et al., 2003. The ore in Nilaterite deposits occurs either as silicates (hydrous silicates such as serpentine, talc, smectite, sepiolite *

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Bauxites and Fe laterites were formed on Neogene volcanics from Fongo-Tongo region in the highlands of western Cameroon, and are distributed on plateaus, slopes and downslope surfaces. Bauxitic profiles result from intense in-situ weathering of trachytes that implied depletion of silica and labile elements from the saprolite, while alumina relatively accumulated from parent minerals pseudomorphosis by primary gibbsite formation. During ongoing lateritization and bauxite maturation, important leaching and illuviation processes resulted in secondary gibbsite crystallizations. Late incision and dissection of upper bauxitic plateau resulted in degradation and dismantling of bauxitic duricrusts with Fe-depletion and increasing silica. Compared to trachyte, bauxitic duricrusts are relatively enriched in Nb, Zr,

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