The Ponza Island bentonite is a mineralogically zoned unit formed by alteration of Pliocene, or younger, rhyoliters. At levels less than 60 m, smectite is the dominant phyllosilicate phase. At greater depth, a mixed-layer illite-smectite dominates. Other main mineral phases are tridymite and sanidine. Quartz, plagioclase feldspar, mordenite and dolomite occur in minor amounts. The change in dominant phyllosilicate phase is associated with a calculated temperature increase from 127°-230°C to 315°-368°C. Chemical calculations indicate that the altering solution was mainly meteoric water with pH values between 6 and 7.5. The bentonite is thus considered to be part of a hydrothermal alteration aureole. Assuming rainfall to be the main source of meteoric water, the deposit may have developed in about 150,000 years.