『Abstract
A laterite profile developed on serpentinite in Lomie
(eの頭に´), south-east Cameroon, was studied to understand
its structure and genesis. Micromorphological, mineralogical and
geochemical characteristics were studied, using thin sections,
SEM-EDS, X-ray diffraction and chemical analyses. In the lower
saprolite, magnesite is dissolved and serpentinite is gradually
replaced by kaolinite, without loss of the lithic fabric (orientation
of kaolinite, distribution of magnetite). This nearly iso-volumetric
weathering, at the scale of the sample, can be considered as in
situ replacement of serpentine by kaolinite. Iron oxide hypocoatings
formed along planar voids, and also smectite formed from solution
in the void system. Congruent dissolution of kaolinites starting
in the mid saprolite eventually led to a collapse of the fabric
and to a chaotic accumulation of fragments of the hypocoatings.
In the overlaying layer, this fabric is even more strongly expressed,
and iron oxides are more concentrated. The weathering profile
shows a severe loss of Mg and Si, and an accumulation of Al, Fe,
Ti and some minor and trace elements. The overlying layers have
fabrics and components different from those recognized for the
saprolite, such as clastic quartz grains and iron oxide nodules
with various lithic fabrics, pointing to an admixture of allochthonous
materials. The appearance of mobile elements (e.g. Rb, Sr, Pb)
and certain stable elements (Zr, Nb), as well as the removal of
serpentinite-related elements clearly show the mixing with materials
derived from micashists in the vicinity. The presence of a discontinuity
in the profile is also confirmed by variations in iron oxide mineralogy.
At least three lithological discontinuities were identified.
Keywords: Micropedology; Serpentinite; Laterite; Geochemistry;
Gibbsite; Ferralsols』
1. Introduction
2. Geographic and geological settings
3. Materials and methods
4. Results
4.1. Parent rock
4.2. Lower saprolite (LS)
4.3. Middle (MS) and upper saprolite (US)
4.4. Laterite level 1 (LL1)
4.5. Laterite level 2 (LL2)
4.6. Laterite levels 3 (LL3) and 4 (LL4)
4.7. Gravel layer (GL)
4.8. Soil cover (SC)
5. Discussion
6. Conclusions
References