Mossbauer spectrometry of natural samples of tourmaline shows the presence of Fe2+ and Fe3+ either in the Y (9b) or the Z (18c) sites, with intervalent iron delocalized between these sites. To understand the distribution of iron in natural samples, tourmaline was synthesized at temperatures from 400 to 700 °C, under a pressure of 100 MPa, by the transformation of an Fe2+-rich natural chlorite in the presence of Na0.5K0.5-feldspar, boric acid and H2O in stoichiometric proportions. The oxygen fugacity fO2, of most experiments was buffered by solid assemblages including hematite (HM), nickel-nickel oxide (NNO) and quartz-fayalite-iron (QFI). It was possible to synthesize tourmaline with Fe2+ in the Z site and Fe2+ or Fe2+ + Fe3+ in the Y site. These site occupancies are similar to those observed in many natural samples. For each fO2 buffer, Mossbauer spectrometry shows that Fe3+/Fe2+ in the Y site is correlated positively with T and fO2 for T <550°C. For higher temperatures, a negative correlation is observed. These relationships reflect structural changes involving the localization of Fe2+ in the Z site and an increasing amount of Al in the Y site. Fe3+/total Fe can be represented by general equations of the type [%Fe3+] = e(a logfO2 + b). The curves corresponding to each temperature are roughly parallel and equidistant. Thus, Fe tourmaline is a potential geothermometer and an fO2 indicator.