African red clays and Keuper Marl are materials of contrasting origin and mineralogical composition. The red clays are residual soils produced by weathering of rocks at the land surface under relatively high temperature and rainfall and good drainage. Bases and silica are leached, leaving the soil rich in iron and aluminium. In Kenya the clay minerals and iron oxide occur respectively as metahalloysite and hematite under relatively dry conditions, and as hydrated halloysite and goethite under moister conditions. The plasticity of the clay minerals is modified by the iron oxide. Black montmorillonitic clays, rich in bases and silica, occur in association with the red clays but on lower ground.
The Keuper Marl is the product of weathering of higher land, which accumulated to a great thickness in a wide basin. Conditions of internal drainage led to enrichment of bases and silica. Calcium and magnesium are present as calcite and dolomite, and silica is present as quartz. All the clay minerals found were rich in magnesium and silica. Illite and chlorite were universal, and much of the chlorite was of the swelling type. In addition, sepiolite and palygorskite commonly occurred. Quantitative x-ray analyses showed up to 90 per cent of clay minerals, but particle-size analyses showed a much smaller proportion of clay-sized minerals, due to particle aggregation or to the occurrence of unusually large clay mineral particles.
- © The Geological Society of London 1967