This study is located on the bentonite outcrop of ‘El Toril’ (Cabo de Gata, Almeria, Spain), where acid solutions have resulted in an alteration front that has caused physical, chemical and mineralogical changes in the bentonite material. The main objective is to assess these changes and to draw a model of evolution of this front in the outcrop. The most important chemical and mineralogical changes observed consist mainly of the loss of mass in the bentonite, the formation of kaolinite, the precipitation of jarosite and alunite from leachates and important changes in the rare earth element distribution pattern. No sulphurs have been identified in the pyroclastic rock at the top of the outcrop, but the presence of dissolution marks and the high concentrations of As, Pb and V in the rock overlying the bentonite point towards the existence, in principle, of these sulphurs and suggest that meteoric alteration is mainly responsible for the genesis of the outcrop.

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