Etame,J., Gerard,M., Suh,C.E. and Bilong,P.(2009): Halloysite neoformation during the weathering of nephelinitic rocks under humid tropical conditions at Mt Etinde, Cameroon. Geoderma, 154, 59-68.

『カメルーンのエティンデ山の湿潤熱帯条件下での霞岩質岩の風化に伴うハロイサイトの新生』


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


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