The Rio Tinto (Tinto river), SW Spain, is one of the most polluted fluvial environment of the world as a consequence of the intense mining and smelting activities developed in its upstream area since prehistoric times. The load suspended in this river is largely composed of clay minerals (mica, kaolinite and chlorite) and inorganic amorphous components (allophane and iron oxide gels) with significant amounts of quartz, feldspars, and locally jarosite, hematite and carbonates. Although clay degradation occurs by the effect of acid mine-drainage, the spatial distribution of clay minerals and associated mineral and amorphous phases is in good agreement with the composition of the rocks and soils drained by the river. Therefore, the mineral assemblages reflect the inherited detrital character of the suspended sediments.