Effects of acid mine drainage on clay minerals suspended in the Tinto River (Rio Tinto, Spain). An experimental approach

The Tinto liver is one of the most polluted stream environments in the worl d, as a result of both acid mine drainage and natural acid rock drainage. T wo representative samples from the phyllosilicate-rich rocks exposed in the drainage basin (Palaeozoic chlorite-bearing slates and Miocene smectite-ri ch marls) were treated with acid river water (pH = 2.2) for different times to constrain the effects of extreme hydrogeochemical conditions on clay mi neral stability. Illite and kaolinite did not show appreciable variations i n their crystal chemistry parameters upon treatment. Chlorite underwent an incipient chemical degradation evidenced by the progressive loss of Fe in o ctahedral positions coupled with a shortening of the b unit-cell parameter, although no weathering products of chlorite were observed. Smectite and ca lcite 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 pha se largely enriched in Al and Mg.