Different authigenic mineral assemblages and mineral compositions have been found in different areas of the Larderello-Travale geothermal field. In the peripheral areas the most abundant authigenic minerals are quartz, calcite, chlorite, and sulfides. They formed in a temperature range of 150 degrees to 250 degrees C and with a P (sub CO 2 ) range of 0.1 to 1.0 atm. In a hotter part of the field, at depths between 200 and 1,300 m, where the main productive fractures are located, the most important authigenic minerals are quartz, chlorite, calcite, K-feldspar, and K-bearing mica, with which wairakite, hematite, anhydrite, and barite are sometimes associated. Deposition of all these minerals occurs in a temperature range of 200 degrees to 300 degrees C under P (sub CO 2 ) of about 1 atm, and relatively high P (sub CO 2 ) , in fluids sometimes characterized by silica oversaturation. Below the 1,300-m depth in the hottest part of the field K-feldspar, chlorite, epidote, sulfides with minor sphene, prehnite, clinopyroxene, and clinoamphibole are present. These minerals form in a temperature range of 250 degrees to 350 degrees C under a P (sub CO 2 ) of less than 1 atm and a P (sub CO 2 ) ranging from 10 (super -40) to 10 (super -33) atm and reasonably reflect the physicochemical parameters of the geothermal fluid before exploitation. Evidence of an earlier hydrothermal stage characterized by higher temperatures can be seen in the occurrence of relic wollastonite and andradite.