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

摘要

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.