『Abstract
A soil chronosequence consisting of six profiles formed on quartz
tholeiite basalt ranging in age from 10,000 years to 1.8 Million
years (My) was studied here. Soil clays were identified using
XRD diffractogram decomposition methods for samples obtained from
the A and C horizons of profiles. The results showed that kaolinite
minerals dominated in all the clay fractions. Gibbsite was prominent
in the C horizons in the soils from older rocks. Clays in the
A horizon of relatively young soils showed an initial stage of
illite formation, followed by smectite mixed layer minerals (illite-smectites
and then vermiculite-illite) and finally by vermiculite. The initial
presence of illite is interesting as there is no magmatic micaceous
or phyllosilicate phase in these basalts and the formation of
illite we attribute to a secondary process, probably created by
alkali transport by plant materials. The change in 2:1 clay mineralogy
reflects the overall change in Si/Al ratios in the soils over
longer periods of weathering. In all cases gibbsite is more abundant
in the C horizons than the A horizons. The difference in gibbsite
content between the A and C horizons we attribute to plant transport
of siliceous phytolite material to the surface. Continued high
rainfall over long periods of time removed the alkali faster than
the plants could bring it to the surface, which led to continuous
lowering of 2:1 minerals from younger to older in the soil chronosequence.
Nevertheless a 2:1, silica-rich mineral persists in the clay assemblages
although in very minor amounts.
Keywords: Basalt; Clay minerals; Silica-rich 2:1 minerals; Soil
chronosequence』
1. Introduction
2. Materials and methods
2.1. Sites and sampling
2.2. Soil chemistry
2.3. Soil mineral identification and grain sizes
3. Results
3.1. Soil characteristics
3.2. Grain sizes
3.3. Clay mineralogy
4. Discussion
4.1. Soil chemistry
4.2. Clay mineralogy
5. Conclusion
Acknowledgements
References