An occurrence of interstratified kaolinite-smectite minerals in a red-black soil toposequence

Abstract

Abstract Interstratified kaolinite-smectite minerals form the major part of a pedon transitional between ‘red’ kaolinitic and black smectitic soils in a soil toposequence derived from basalt in Burundi, Central Africa. These interstratified kaolinite-smectites have many X-ray, morphological and chemical features in common with ‘tabular halloysite’. Modifications to the 00 l spacings of these minerals brought about by thermal and chemical treatments, however, show that they are not halloysite but consist of a mixture of ∼70% kaolinite and 30% smectite randomly interstratified. Field evidence indicates that these clays can be considered as a transient stage in the overall reaction smectite→kaolinite + iron oxides, which takes place in this landscape whenever the slope gradients give rise to better drainage conditions.