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The weathering of some granitic rocks in the West Region of Cameroon (Central Africa) was examined by X-ray diffraction (XRD) and X-ray fluorescence Spectrometer (XRF) in conjunction with Scanning Electron Microscopy (SEM). Kaolinite and illite were the most predominant weathering products whereas gibbsite, goethite and maghemite were also found in small amounts; only residual quantities of quartz and potassium feldspar were observed. The geochemical study shows the mobility of major elements during the progressive kaolinization of granitic rocks. The main trend is the loss of elements, with high losses of CaO, Na 2 O and MgO, slight losses of SiO 2 while Al 2 O 3 and K 2 O appear to increase with weathering intensity. Chemical index of alteration (CIA) shows a high value (68-95%). The scanning electron microscope analysis reveals the existence of several surface features that reflect the effect of mainly chemical actions on the quartz and K-feldspar grains. Quartz and K-feldspar crystals from kaolinized granitic rocks show low to very high chemical etching attributed to weathering under moderate to severe environmental conditions such as low pH, high temperature and humidity and extreme leaching typical of tropical climatic conditions. Etched pit densities on quartz surfaces increase with weathering, as a result of increasing silica concentrations in pore fluids.

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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The present study was conducted on lateritic sub-grade soils with the pavement structure built using lateritic materials. The objective of this work was to characterize the lateritic materials and obtain a correlation between the modulus of elasticity (E dyn) and the Californian bearing ratio (CBR) around the Mbu-Baforchu area in a bid to dimension a second access highway to the city of Bamenda. Identification tests (water content, particle size analysis, Atterberg limits) and mechanical characteristics tests (Modified Proctor, CBR and simple compression) were carried out on soil samples taken from the field. The CBR results were variable with values between 32 and 49 for burrow materials, while those for sub-grade vary between 6.5 and 14.4. A regression equation between the E dyn and CBR for the burrow material gave E dyn = 155.8exp (0.0541CBR) with a regression coefficient of 0.9599 while that for the sub-grade material was E dyn = 69.064exp (0.0706CBR) with a regression coefficient of 0.9295. The burrow and sub-grade soils fall within the S3 and S5 soil classes respectively. The HRB classification pits the burrow materials in the A-2 class with a group indices varying between 0 and 6, which describes them as a clayey loamy soils with (compressible nodular silty sands), while the sub-grade soils falls in the A-7-5 class and having variable group indices (12-20), thereby characterizing them as inorganic clays of high plasticity. The pavement structure designed for consists of a bituminous cover of 5 cm, a base course of 20 cm and a sub-base course of 20 cm. The pavement structure can carry a traffic loading of up to 3000 vehicles per day, inducing stresses and deflections within the pavement structure which are lower than their required permissible values. With ε t = − 14.5 μdef, ε t, perm = 62.8 μdef for the riding surface; σ t = − 0.118 MPa with σ t, perm = 0.427 MPa representing the permissible value for the base layer and ε z = 103.8 μdef, ε z, perm = 487.3 μdef for the sub-base layer; which reassures us that the pavement structure is durable.

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