Morphological, mineralogical and geochemical characterization of Ngaoundal soils (Adamawa-Cameroon): Implications for weathering mechanisms and trajectories in tropical zones

Abstract

• These lateritic soils studied are silico-ferrugino-aluminous. • Their silica/sesquioxide ratios correspond to those of true laterites (S/R < 1.33). • The laterites studied are characterised mainly by minerals resistant to weathering. • The predominant pedogenetic process in the Ngaoundal laterites is ferugination. • These soils are moderately to strongly lateritic, while some samples are kaolinitised. Ngaoundal, located in the Adamaoua region of Cameroon, has a tropical climate and is known for its characteristic soils. The aim of the present study was to evaluate the action of weathering on Ngaoundal's lateritic soils. Morpho-structural, mineralogical and geochemical analyses enabled us to carry out this study. The morphological study carried out on 15 soil samples shows a variation in colour from reddish (2.5YR 5/8) to dark brown (7.5YR 6/8), varying from reddish yellow (7.5YR 6/8), orange brown (10YR 5/6) and yellowish brown (10YR 5/4), and have clayey to sandy-clayey matrices with nodules of the order of millimetres to centimetres. X-ray diffractometry and x-ray fluorescence show the following mineral phases in Ngaoundal's lateritic soils: quartz, goethite, gibbsite, kaolinite, hematite, magnetite, anatase and boehmite. Geochemical analysis of these soils shows high average levels of SiO 2 (36.41 %), Fe 2 O 3 (31.48 %) and Al 2 O 3 (17.72 %), and low average levels of TiO 2 (1.02 %) and CaO (0.36 %), which characterise characteristic soils rich in ferruginous oxide and alumina oxide. The weathering indices of Ngaoundal soils (sesquioxide ratio) vary between 0.22 and 2.98 with an average of 1.09. This average value qualifies these soils as lateritic soils rich in gibbsite and kaolinite (average CIA = 96.38 %) and resulting from chemical alteration of the hydrolysis type (alloying and monosiallitisation).