Abstract
Three soil profiles on the basement rocks in Awa-Oru-Ijebu Igbo area of southwestern Nigeria were investigated for the formation secondary minerals and the comparison of the mineralogical and geochemical patterns in the weathering profiles overlying the rocks during the humid tropical weathering. X-ray data showed that the primary mineral constituents of the rocks such as feldspar, biotite, hornblende have altered to kaolinite and hematite. In soil horizon, kaolinite and quartz are the dominant minerals. The concentration of hematite in the laterite indicated that most of the iron oxides generated in the soil horizon have been leached into the underlying laterite layer. Kaolinite is the dominant mineral in the clayey horizon and its presence suggests that some of the iron bearing ferromagnesian minerals have altered into the kaolinite. Geochemical data further revealed significant enrichment of Al2O3 and Fe2O3 in the laterite profile compared with the parent rock, while, CaO, Na2O, K2O, P2O5 and MnO show strong depletion up the profiles. The removal of mobile elements by meteoric water and subsequent concentration of stable weathering products results in lateritization. The absence of bauxite minerals like gibbsite and diaspora in the profiles shows that the trend of weathering is towards iron enrichment (ferralitization) and not aluminum accumulation (bauxitization). The CIA values of the lateritic layers shows that the weathering process is matured and has reached advanced stage.Keywords: Basement rocks, weathering, lateritization, kaolinite, Hematite.
Highlights
Laterites and lateritic soils abound in tropical and subtropical regions of the world
On tropical African shields, lateritization processes have led to the development of weathering mantles ten of meter thick often capped with ferricretes at the expense of all kinds of parent rocks that may reflect a long and complex evolution including climatic geomorphological changes (Aleva 1994)
The weathering profiles over pegmatite, granite gneiss and granite are residual because, on the field, there is no evidence of movement of the soil except for the local down slope wash that characterized the base of the profiles above
Summary
Laterites and lateritic soils abound in tropical and subtropical regions of the world. On tropical African shields, lateritization processes have led to the development of weathering mantles ten of meter thick often capped with ferricretes at the expense of all kinds of parent rocks that may reflect a long and complex evolution including climatic geomorphological changes (Aleva 1994). Such lateritization processes commonly involve preferential leaching and concentration of valuable deposits, such as ores of aluminium, iron, nickel, chromium, cobalt, manganese, gold, tin, niobium and tantalum. The clayey residues generated from the chemical decomposition of rocks within these areas are sometimes found to be suitable for industrial applications
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