Changes in physical properties of Fe-amphiboles at increasing T have been proven to be crucial in modeling the electrical conductivity of subducted rocks in convergent margins (Reynard et. al., 2011). Oxidation of Fe2+ to Fe3+ within the octahedral sites, which in turn is aimed at achieving local electroneutrality during deprotonation occurring at high temperature (HT), is considered to be the reason for the observed increase in conductivity (e.g. Wang et al., 2012). Deprotonation mechanisms and their relationship with Fe oxidation within the amphibole structure are now being examined systematically combining single-crystal structure refinement with FTIR and Mössbauer spectroscopies. In particular, HT-FTIR spectroscopy has proved to yield in operando information concerning deprotonation and, in favorable conditions, oxidation processes of multi-valence octahedral constituents. The interpretation of FTIR data has been repeatedly tested by single-crystal structure refinement. A riebeckite from Malawi was chosen for this work as a representative of the subgroup of the sodium amphiboles, which are common in high-pressure (HP) geological environments. Before studying its HT behavior, the sample was exhaustively characterized at room T (RT) by means of single-crystal XRD structure-refinement, EPMA, IR and Mössbauer spectroscopy. Its crystal-chemical composition turned out to be very close to that of the ideal end-member, i.e. A?BNa2C(Fe2+3Fe3+2)TSi8O22W(OH)2. Interestingly, HT-IR data collected in-situ using short-term heating ramps on single-crystals showed a significant (apparent?) increase in the absorption coefficient with T, a feature already observed on potassic-ferro-richterite by Della Ventura et al. (manuscript in preparation). Spectra collected at RT during the ramp after fast quenching showed that the OH content does not change up to 550°C and that hydrogen is rapidly lost in the 550-650°C T interval. Doubly-polished single crystals and powders were studied by HT-FTIR during long-term isothermal annealing at several T values with the aim of studying the effects of H diffusion through the crystal on deprotonation kinetics. Notably, all experiments done on powders did not show any apparent increase in the absorption coefficient at high T, suggesting that such a phenomenon is relevant to single crystals only. Experiments done under isothermal conditions and different chemical/physical environments showed that the kinetics of deprotonation is strongly related to oxygen availability during the heating stage. In particular, deprotonation was found to be strongly inhibited both in single crystals heated under a N2 flux and in powders embedded in a KBr pellet. Attempts to quantify and model the impact of the medium on hydrogen diffusion in riebeckite are still under way
HT-FTIR spectroscopy of riebeckite
2015
Abstract
Changes in physical properties of Fe-amphiboles at increasing T have been proven to be crucial in modeling the electrical conductivity of subducted rocks in convergent margins (Reynard et. al., 2011). Oxidation of Fe2+ to Fe3+ within the octahedral sites, which in turn is aimed at achieving local electroneutrality during deprotonation occurring at high temperature (HT), is considered to be the reason for the observed increase in conductivity (e.g. Wang et al., 2012). Deprotonation mechanisms and their relationship with Fe oxidation within the amphibole structure are now being examined systematically combining single-crystal structure refinement with FTIR and Mössbauer spectroscopies. In particular, HT-FTIR spectroscopy has proved to yield in operando information concerning deprotonation and, in favorable conditions, oxidation processes of multi-valence octahedral constituents. The interpretation of FTIR data has been repeatedly tested by single-crystal structure refinement. A riebeckite from Malawi was chosen for this work as a representative of the subgroup of the sodium amphiboles, which are common in high-pressure (HP) geological environments. Before studying its HT behavior, the sample was exhaustively characterized at room T (RT) by means of single-crystal XRD structure-refinement, EPMA, IR and Mössbauer spectroscopy. Its crystal-chemical composition turned out to be very close to that of the ideal end-member, i.e. A?BNa2C(Fe2+3Fe3+2)TSi8O22W(OH)2. Interestingly, HT-IR data collected in-situ using short-term heating ramps on single-crystals showed a significant (apparent?) increase in the absorption coefficient with T, a feature already observed on potassic-ferro-richterite by Della Ventura et al. (manuscript in preparation). Spectra collected at RT during the ramp after fast quenching showed that the OH content does not change up to 550°C and that hydrogen is rapidly lost in the 550-650°C T interval. Doubly-polished single crystals and powders were studied by HT-FTIR during long-term isothermal annealing at several T values with the aim of studying the effects of H diffusion through the crystal on deprotonation kinetics. Notably, all experiments done on powders did not show any apparent increase in the absorption coefficient at high T, suggesting that such a phenomenon is relevant to single crystals only. Experiments done under isothermal conditions and different chemical/physical environments showed that the kinetics of deprotonation is strongly related to oxygen availability during the heating stage. In particular, deprotonation was found to be strongly inhibited both in single crystals heated under a N2 flux and in powders embedded in a KBr pellet. Attempts to quantify and model the impact of the medium on hydrogen diffusion in riebeckite are still under wayI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
