The Tinto river is one of the most polluted stream environments in the world, as a result of both acid mine drainage and natural acid rock drainage. Two representative samples from the phyllosilicate-rich rocks exposed in the drainage basin (Palaeozoic chlorite-bearing slates and Miocene smectite-rich marls) were treated with acid river water (pH = 2.2) for different times to constrain the effects of extreme hydrogeochemical conditions on clay mineral stability. Illite and kaolinite did not show appreciable variations in their crystal chemistry parameters upon treatment. Chlorite underwent an incipient chemical degradation evidenced by the progressive loss of Fe in octahedral positions coupled with a shortening of the b unit-cell parameter, although no weathering products of chlorite were observed. Smectite and calcite were rapidly and fully dissolved thus neutralizing the water acidity, and subsequently Fe and Al oxy-hydroxides and opaline silica precipitated from the aqueous solution, together with a neoformed amorphous silicate phase largely enriched in Al and Mg.