A widespread Quaternary hydrothermal event in Central Italy was responsible for large metalliferous mineralizations which include manganese oxides, pyrite, other sulphide phases and several clay minerals. The deposits occur in Carboniferous-Permian metasedimentary rocks of the Verrucano Formation near Capalbio, Grosseto Province, Tuscany. Among the clay minerals, the presence of halloysite with 10 Å and 7 Å terms is particularly important along with that of smectite and mixed-layer minerals of the I/MO. All the aspects of the process can be pointed out by following the muscovite alteration phases: muscovite and/or sericite-> illite-> illite/smectite-> smectite-> 10 Å-halloysite-> 7 Å-halloysite.