Geochemical balance of lateritization processes and climatic signatures inweathering profiles overlain by ferricretes in Central Africa

A simple geochemical balance of lateritization processes governing the deve lopment of several tens of meters of weathering profiles overlain by ferric retes is estimated on the basis of detailed mineralogical and geochemical d ata. The lateritic weathering mantle of the,"Haut-Mbomou" area in Central A frica is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite, a mottled clay layer, a soft nodular layer, a soft ferricrete, and a ferricrete in which kaolinite, gibb site, goethite, and hematite occur in various quantities. Incongruent disso lution of kaolinite leads to the formation of gibbsite in the upper saproli te, whereas the hematite does not clearly replace the kaolinite according t o an epigene process in the upper ferruginous layers of the profiles. Inste ad, that kaolinite is also transformed into gibbsite according to an incong ruent dissolution under hydrated and reducing conditions induced by a relat ively humid climatic pattern. The respective relations of the silica, iron, and aluminum balances and the Al substitution rate of the hematite on the one hand, and of RHG [RHG = 100 (hematite/hematite + goethite)] and the kao linite on the other hand, to the consumption or the release of protons H+ p ermit differentiation of aggrading ferruginization and degradation processe s operating in the different lateritic weathering profiles. The Al substitu tion rate of the Fe-oxyhydroxides varies according to the nature of laterit ization processes, e.g., saprolitic weathering and aggrading ferruginizatio n vs. degradation The observations and results indicate that the ferruginiz ation process of the weathering materials of parent rocks is not a simple o ngoing process as often thought. This suggests that the actual lateritic we athering:mantle of the Haut-Mbomou area may result from different stages of weathering and erosion during climatic changes. Copyright (C) 1999 Elsevie r Science Ltd.