MINERALOGY AND GENESIS OF CLAYS IN RED‐BLACK SOIL TOPOSEQUENCES ON BASIC IGNEOUS ROCKS IN KENYA

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Summary: The clay mineralogy of two red‐black soil toposequences on basic igneous rocks in Kenya has been investigated. Besides some illite and amorphous material kaolinites predominate in the slope soils (ultisols) whereas smectites are the main clay minerals in the soils of the depressions (vertisols). The kaolinites are poorly crystalline and have high surface area. According to chemical analysis, IR spectroscopy, and D.T.A. the smectites appear to be a ferriferous member of the montmorillonite‐beidellite series having approximately 0·5 Fe 3+ in octahedral and 0·2 Al per O 10 (OH) 2 in the tetrahedral position. The profile and slope distribution of kaolinites and smectites led to the conclusion that the smectites are the first weathering product and, depending on hydrological conditions governing the soil solution composition (Si, Mg, pH), either persist (depression) or are decomposed (slope) and followed by kaolinites.

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