We performed XRD investigations on the sedimentary cap rock of the geothermal system developed in the area of Vico volcano (Northern Latium) to assess its effectiveness and degree of interaction with fluids. The system consists of a positive thermal anomaly, a permeable carbonate reservoir at shallow depths and a low permeability siliciclastic cap rock. Unfractured cap rock shows maximum paleo-temperatures <50-60 degrees C, interpreted as the thermal signature of the original sedimentary basin. Fractured cap rock is characterized by kaolinite, calcite, short-range ordered mixed layers illite-smectite with paleo-temperatures between 85 and 110 degrees C indicating strong interaction with hot fluids from a carbonate reservoir. (C) 2013 Elsevier Ltd. All rights reserved.