The oxidation of Fe in riebeckite at 0.7 GPa

At high temperatures, riebeckite, ideally Na2(Fe32+Fe23+)Si8O22(OH)2, undergoes significant crystal-chemical rearrangement in response to the following reaction: Fe2+ + OH− → Fe3+ + O2− + e− + H+, leading to the development of charge carriers (hopping electrons and delocalized H+ ions) and to the release of H2O. The process of oxidation of Fe and dehydrogenation thus has important implications for planetary-scale phenomena, including electrical conductivity, water cycling, seismicity, volcanism, and ore generation, particularly in subduction zones. In this work, we address the effect of pressure on the oxidation of Fe in riebeckite, a process that is irreversible for temperatures above ∼ 377 °C at 1 atm pressure, under oxidizing conditions. Single crystals of riebeckite were annealed using a piston-cylinder apparatus at temperatures (T) in the 600–750 °C range under a constant pressure (P) of 0.7 GPa and oxygen fugacity (fO2) close to Ni-NiO (NNO). The crystal-chemical response of the amphibole to these P–T–fO2 conditions was analyzed using a combination of X-ray diffraction and spectroscopic methods, including Mössbauer, Raman, and Fourier transform infrared (FTIR) spectroscopy. At 0.7 GPa, there is no oxidation of Fe in riebeckite. The stability of the amphibole structure extends at least up to 750 °C, without any transition to oxo-riebeckite and hence without any release of H+. Since the conditions of our experiments replicate those of rocks at pressures corresponding to ∼ 25 km depth, our findings further support the concept that thermally activated electron hopping significantly contributes to the electrical conductivity of lithospheric rocks, and we suggest that both conductivity and water cycling in the Earth's interior may be active over a broader P–T range (and thus at grater depths) than previously considered.