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.

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

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, Na2O and MgO, slight losses of SiO2 while Al2O3 and K2O 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.

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The Origin of Etch Pits Recorded on Residual Grain Surfaces from Kaolinized Granitic Rocks West Region Cameroon

Soils developed on garnet-rich micaschist in the Southern Cameroon plateau were investigated. Morphologically, two weathering materials constituted the study profile, fine earth and nodules. Each material has a singular evolution from the coarse saprolite to the upper set of clayey and loose horizons. There is Al2O3 enrichment in the fine earth concomitantly to Fe2O3, Na2O and K2O enrichment in nodules in major element–Al2O3 variation diagrams. A–CN–K and A–CNK–FM diagrams show a progressive depletion of Al in nodules concomitantly to an enrichment of this element in the fine earth. Three pedogenetic processes emerge from the evolution of weathering products. There is a ferruginization process whose importance increases from the coarse saprolite to the upper set of clayey and loose horizons, giving rise to the formation of dense nodules composed of hematite, goethite and gibbsite. This ferruginization occurs concomitantly with a desaluminization process, which causes Al2O3 depletion in nodules. These nodules are formed by the process of induration where the hydrated amorphous hydroxide and oxides are dehydrated to their crystalline oxide form, due to condensation. In addition, there is aluminization, which occurs in the fine earth, leading to the development of a red clayey material composed of kaolinite and gibbsite.

/pdf/tropical-chemical-weathering-of-a-garnet-rich-micaschist-in-4uqzf8f9ha.pdf

Tropical chemical weathering of a garnet rich micaschist in the rainforest zone of Cameroon.

https://link.springer.com/content/pdf/10.1007/s42452-021-04837-5.pdf

Potential use of alluvial clays from Monoun in ceramics by adding feldspar from Batie (West-Cameroon) as a fluxing agent

Mineralogical study of weathering on the bana complex,western part of cameroon(カメルーン国西部バナ複合岩体の風化に関する鉱物学的研究)

The weathering process of the Pan African granitic rock of Batie was mineralogically investigated by means of Microprobe analysis (EPMA), X-ray diffraction (XRD), X-ray fluorescent (XRF), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The porphyritic granite is composed of biotite, feldspar and quartz as main minerals, whereas epidote, chlorite, kaolinite and mica clay mineral are secondary products. Mica clay mineral and kaolinite are main constituent clay minerals in the granite saprolite (weathered granite). Mica clay mineral predominantly occurs at the lower part of the investigated profile associating with a small amount of kaolinite. The granite saprolite at the top part of the profile consists mainly of platy kaolinite with a small amount of lath shape mica clay mineral. Mica clay mineral was formed in the hydrothermal clay veins occurring in the weathering profile. It is inferred that mica clay mineral occurred in the altered granite was formed by hydrothermal activity. Detailed observations of morphological variation by TEM with X-ray analysis revealed that crystal size and shape reasonably explain the transformation sequence of kaolinite as weathering proceeds. Close relationship between crystal size and morphology is obtained. Etch pits developed on quartz and K-feldspar grains are commonly observed. However, shape and size of the etch pits are quite complicated suggesting none monotonous micro-weathering condition. Etch pits found in the upper part are much rougher than those of the lower part. Based on the results obtained, weathering condition of the Batie granite was discussed mainly from the standpoint of clay mineralogy.

Kaolinization of saprolite developed on pan african granite of batie district, west province of cameroon

The weathering mechanism of pyroclastic rocks distributed at the landslides in Lembo area, Bana-Bangou district, was investigated from a mineralogical viewpoint. Specimens collected from the landslide profile were composed mainly of kaolinite, illite, chlorite, illite-smectite interstratified mineral, gibbsite, goethite, anatase, maghematite, hematite, quartz and feldspar. The landslide area (pyroclastic rock) was characterized by slow creep movement every raining season. The slip zone was recognized on the landslide profile. Chemical composition of the landslide materials showed the complete leaching of CaO and Na 2 O, whereas a small amount of K 2 O and MgO remained in the whole profile. Detail examinations of the microtexture indicated that the material showed aggregate skeletal matrix with high porosity. This process further caused the decrease of soil strengths and development of micro-texture characteristics that tended to increase the creeping landslide. This displacement rate of the creeping landslide mechanically depended mostly on the pore water pressure in the landslide profile. High pore water pressure always corresponded to the large displacement rate. The mechanism of the creeping landslide was considered from mineralogical characteristics.

Tchoua Felix Mbakam

Papers

Kaolinization of saprolite developed on pan african granite of batie district, west province of cameroon

Clay mineralogy of the landslide in lembo area, bana-bangou district, west province of cameroon