Clay mineralogy in southern Africa river muds M. SETTI 1, *, A. LO ´ PEZ-GALINDO 2 , M. PADOAN 3 AND E. GARZANTI 3 1 Dipartimento di Scienze della Terra e dell‘Ambiente, Universita ` di Pavia, Via Ferrata 1, 27100 Pavia, Italy, 2 Instituto Andaluz de Ciencias de la Tierra. CSIC-UGR. Avenide de las Palmera 4, 18100 Armilla, Granada, Spain, 3 Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Universita ` di Milano-Bicocca, Piazza della Scienza 4, 20216 Milano, Italy (Received 10 July 2014; revised 30 October 2014; Editor: Eric Ferrage) ABSTRACT: The composition, morphology and crystal order of clay minerals in silt-sized sediments carried in suspensions from 25 major rivers across tropical southern Africa have been studied by X-ray diffractometry and scanning and transmission electron microscopy. Our goal was to determine the spatial variability of clay-mineral associations in diverse geological settings, and in climatic conditions ranging from humid Angola and Zambia to hyperarid Namibia and the Kalahari. Specific attention was paid to the micromorphology and chemical composition of smectite particles. The relative abundance of smectites, illite/mica, kaolinite and chlorite enabled identification of regions characterized by different physical and chemical processes: (1) negligible chemical weathering is documented in Namibia, where river muds mostly contain illite/mica or smectite derived from Damara metasedimentary or Etendeka volcanic rocks; (2) kaolinite documenting intense weathering, reaches a maximum in the Okavango, Kwando and Upper Zambezi, sourced in subequatorial Angola and Zambia; (3) suspended-load muds in the Limpopo and middle Zambezi catchments display intermediate features, with varied assemblages and smectite compositions reflecting diverse parent lithologies. Clay mineralogy and chemical composition are confirmed as a most effective tool to unravel present and past climatic conditions on a continental scale. KEYWORDS: clay minerals, chemical weathering, Zambezi River, Limpopo River, Okavango rivers, South Africa. Rivers are the main transport mechanism of detrital material into the oceans, which represents 70% of the sedimentary matter (Lisitzin, 1972). The composition and textural characteristics of the transported particles depend both on the types of rock eroded by the rivers and the climate in the catchment area. During transport, mineral particles generally do not undergo significant chemical changes, in most cases remaining unchanged. However, the mineral associations may vary significantly along of the river, depending on the geological characteristics of the areas traversed and the sediments provided by its tributaries. The composition of fine-grained sediments transported by large rivers worldwide has been studied extensively (Emeis & Stoffers, 1982; Emeis, 1985; Konta, 1985; Johnsson et al., 1991; Hay, 1998; Zabel et al., 2001; Gaillardet et al., 1999; Viers et al., 2009). Clay minerals predomi- nate in river-suspended solids and chiefly reflect the composition of soils in the drainage basins. The type of clay mineral assemblages, the morphology of clay particles and their chemical composition are normally used for climatic interpretations (see Biscaye, 1965; Chamley, 1989; Thiry, 2000, and references therein) taking into account the lithology of the parent rocks, the morphology (topography) of the area and the intensity of erosion. Mica-illite represents the principal and omnipresent phyllosilicate, and is particularly * E-mail: massimo.setti@unipv.it DOI: 10.1180/claymin.2014.049.5.08 Clay Minerals, (2014) 49, 717–733 # 2014 The Mineralogical Society