Kinetics of dehydrogenation of riebeckite Na₂Fe₂³⁺Fe₃²⁺Si₈O₂₂(OH)₂:An HT-FTIR study

In this work, we address the kinetics of dehydrogenation occurring at high temperatures (HT) in riebeckite, a sodic amphibole with the ideal composition Na2Fe23+Fe32+Si8O22(OH)2.${\rm{N}}{{\rm{a}}_2}{\rm{Fe}}_2^{3 + }{\rm{Fe}}_3^{2 + }{\rm{S}}{{\rm{i}}_8}{{\rm{O}}_{22}}{({\rm{OH}})_2}.$We performed isothermal experiments on both powders and single-crystals up to 560 °C and monitored the O-H stretching signal by Fourier transform infrared (FTIR) spectroscopy. Single-crystals show an initial increase in IR absorption intensity due to increasing vibrational amplitudes of the O-H bond stretching, not observed for powders. The OH-intensities vs. time were fitted using the formalism for first-order reactions. The calculated activation energies for H+ diffusion in riebeckite are 159 ± 15 kJ/mol for powders and 216 ± 20 kJ/mol for single crystals, respectively. The exponential factor m in the Avrami-Erofeev equation obtained for crystals ranges between 1.02 and 1.31, suggesting that, unlike powders, the dehydration process in crystals is not a purely first-order reaction. This implies that a second energy barrier must be considered, i.e., diffusion of H+ through the crystal. FTIR imaging showed that H+ diffusion occurs mainly perpendicular to the silicate double-chain. Our results confirm that the release of H+ from riebeckite occurs after the irreversible Fe2+-to-Fe3+ exchange, thus at temperatures >550 °C. To be effective, the process needs the presence of external oxygen that, by interacting with H+ at the crystal surface, triggers the release of H2O molecules. This implies that oxidizing conditions are required for the amphibole to be an efficient water source at depth.