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). We performed isothermal experiments on both powders and single-crystals up to 560 degrees 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 FE 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 AvramiErofeev 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 FE from riebeckite occurs after the irreversible Fe2+-to-Fe3+ exchange, thus at temperatures >550 degrees C. To be effective, the process needs the presence of external oxygen that, by interacting with FE 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.