Evaluation of letsoku and related Southern African clayey soils

Abstract

The nature of letsoku and related clayey soils, traditionally used by indigenous Southern African communities for a wide range of purposes, was explored. Thirty nine samples were collected from Botswana, Lesotho, Swaziland, South Africa and Zimbabwe. They were analyzed to determine their composition and physical properties. Analyses involved BET surface area determinations, pH measurements, X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Structured interviews were used to establish the purpose of use and the location of sourcing sites. Most of the samples were in powder form and some were supplied as dry clay balls. Cosmetic applications were almost universally indicated. However, other functions, related to artwork, medicinal use, cultural symbolism and traditional beliefs were also mentioned. The letsoku samples covered a wide range of colors ranging from bright red to yellow but also from off-white to black with some having a light grey color. It was therefore not surprising that the mineral composition of the letsoku samples also varied widely. A black sample, and the yellow and reddish pastel colored samples, contained significant quantities of the corresponding, color imparting, iron oxides. As expected, clay minerals featured prominently although kaolinite was more often encountered than smectites as the dominant minerals. All samples contained silica and in some instances the content exceeded 90% m/m SiO2. The presence of high contents (40% m/m) of gibbsite in samples from Venda represents a new finding for clayey soils in traditional usage.