La bentonite di Serra Narbonis, Villanovatulo (Sardegna Centrale).

Bentonite is widespread throughout Sardinia. Deposits occur as stratiform mineral bodies (in situ-altered) within poor welded pumice-rich and ash-rich layers, and also as distal fall-out. Other deposits are interlayered within the Miocene sedimentary sequences as lenticular bodies. The former originated by deuteric alteration, and formation is mainly controlled by lithology, the latter type originated by weathering, which altered the volcanic ash-lenses deposited in lacustrine and/or fluvial-deltaic environment). The Serra Narbonis bentonites belong to the latter tipology. Mineral characterization performed on collected samples show that deposit mainly consist of illite, smectite-group minerals, chlorite, and K-feldspars. Serra Narbonis fall-out deposit is formed by clay-rich material, due to the original composition of the lenses. Chemical composition of materials show high content in Ca-Na-Mg. Field and microscope observations suggest that high Fe2O3 contents (2 to 8 wt.%) can be attributed to biotite and/or to iron re-circulation (Fe-oxides and hydroxides)