Fibrous clay minerals in the soils of Namaqualand, South Africa: characteristics and formation

Abstract

Palygorskite- or sepiolite-containing soils occur at elevations up to 1000 m, in gently sloping landscapes formed by aeolian sands or by strongly eroded pedisediments overlying the Namaqualand Metamorphic Complex. The unconsolidated sands are in various stages of calcretization. Most of the soils are sandy, shallow, non-saline, with a pH ranging between 7.2 and 8.2. Carbonate contents are low, but increase towards the calcretized horizons. Most of the carbonates appear in the form of nodules. In the clay fractions, palygorskite or sepiolite dominate and are accompanied by illite and non-expanding 14 Å minerals, or mixed-layer minerals. DTA, TGA and chemical analysis indicate that the sepiolite is Fe-rich and contains organic material trapped within its channels. The highest concentrations of the fibrous clay minerals commonly occur in the bottom horizons. It is postulated that they formed pedogenically from silica released by silicates and Mg derived from Mg-rich carbonates introduced by aeolian accretion. This hypothesis is supported by a decrease in Mg Ca of the carbonates with soil depth. Another possible Mg source considered is sea-spray. Soil-water extract compositions are not compatible with fibrous clay mineral stability. Electron micrographs of sepiolite clays suggest alteration into sheet clay minerals.