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Kinetics of dehydrogenation of riebeckite Na2Fe3+2Fe2+3Si8O22(OH)2: an HT-FTIR study

DOI: 10.2138/am-2022-8021 DOI Help

Authors: Giancarlo Della Ventura (Università di Roma Tre; INFN-Laboratori Nazionali di Frascati; INGV) , Francesco Radica (Università di Roma Tre; INFN-Laboratori Nazionali di Frascati) , Federico Galdenzi (Università di Roma Tre; INFN-Laboratori Nazionali di Frascati) , Umberto Susta (Università di Roma Tre) , Gianfelice Cinque (Diamond Light Source) , Mariangela Cestelli-Guidi (INFN-Laboratori Nazionali di Frascati) , Boriana Mihailova (Universität Hamburg) , Augusto Marcelli (INFN-Laboratori Nazionali di Frascati; Rome International Centre for Material Science Superstripes - RICMASS)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: American Mineralogist

State: Published (Approved)
Published: May 2021
Diamond Proposal Number(s): 11425

Abstract: In this work, we address the kinetics of dehydrogenation occurring at high temperatures (HT) in riebeckite, a sodic amphibole with the ideal composition Na2Fe3+2 Fe2+3Si8O22(OH)2. We did 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.

Journal Keywords: riebeckite; HT-FTIR spectroscopy; FTIR imaging; Fe-oxidation; dehydration kinetics; activation energy

Subject Areas: Earth Science, Chemistry

Instruments: B22-Multimode InfraRed imaging And Microspectroscopy

Added On: 07/05/2021 08:24

Discipline Tags:

Earth Sciences & Environment Mineralogy Chemistry Geology Geochemistry

Technical Tags:

Spectroscopy Infrared Spectroscopy Synchtron-based Fourier Transform Infrared Spectroscopy (SR-FTIR)