『Abstract
Mt Etinde eruptions are dated at 0.65 Ma. It is located on the
SW oceanic border of Mt Cameroon, and is composed essentially
of nephenilitic rocks. The weathering of nephelinites has resulted
in the formation of a 150 cm deep Andic Cambisol (Humic) consisting
of three horizons: Ah/Bw/C. Mineral weathering was studied in
detail in two profiles: one (P1) developed
from unaltered nephelinite (nepheliniteU) and the other
(P2) formed from hydrothermally altered nephelinite
(nepheliniteH). Emphasis was on the impact of hydrothermal
alteration on weathering profiles, to characterise the signature
of halloysite in nephelinitic regoliths and to discuss the evolution
of Mt Etinde soils on the Quaternary nephelinitic parent rocks.
The mineralogy of nepheliniteU consists predominantly
of clinopyroxene, nepheline, leucite, hauyne（uの頭に¨）,
titanomagnetite, perovskite, apatite and sphene. NepheliniteH
differs from nepheliniteU by phillipsite-calcite occurrence.
The main weathering products are halloysites (1-nm and 0.7-nm)
and minor occurrence of gibbsite and hematite. SEM micromorphological
examination coupled with microprobe analyses showed halloysites
resulting from in situ alterations of feldspathoids, phillipsites
and clinopyroxenes. 1-nm hydrated halloysite neoformation characterises
the alteromorphs of the saprolite while 1-nm and 0.7-nm halloysite
coexist in the Bw and the Ah horizons of both soils. During Weathering,
these halloysites had a chemical dependence with parent mineral
as follows: Ce-rich halloysite characterises alteromorph after
phillipsite; Fe-rich halloysite, alteromorph after clinopyroxene;
Ca-rich halloysite alteromorph, after hauyne; and K-rich halloysite,
alteromorph after leucite. Ce-rich halloysite is a tracer of the
hydrothermal impact in soils profile. The Si/Al ratio is also
characteristic of the parent mineral. Therefore, the availability
of Si is the only factor that controlled the formation of 1-nm
halloysite at the base of the saprolite while the presence of
1-nm and 0.7 halloysite in the upper part of the saprolite, the
Bw horizon and the Ah horizon suggesting only time controls of
the evolution of the profiles by both kinetic (wet-dry seasons)
and thermodynamic (Si-bearing) factors. Gibbsite occurs only in
the nepheliniteH soil. Its presence is due to the hydrothermal
alteration which favoured the development of an intense microporosity
allowing the elimination of silica. The study highlighted the
influence of past hydrothermal activity on the recent weathering
process in the humid tropical area.
Keywords: Humid tropical area; Nephelinite; Hydrothermalism; Weathering;
Halloysite』
1. Introduction
2. Study area
3. Materials and methods
4. Results
4.1. Halloysite alteromorphs after feldspathoids, clinopyroxene
and phillipsite
4.2. Halloysite evolution and associated secondary minerals
5. Discussions
5.1. Occurrence of halloysite
5.2. Hydrothermal impact on weathering profiles and pedoclimate
of the Mont Etinde soils
6. Conclusions
Acknowledgements
References