A. Beauvais, Geochemical balance of lateritization processes and climatic signatures inweathering profiles overlain by ferricretes in Central Africa, GEOCH COS A, 63(23-24), 1999, pp. 3939-3957
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.