Fe-amphiboles play a major role in enhancing the electrical conductivity of subducted rocks in convergent margins (Reynard et.al., 2011). This issue has been shown to be related to the Fe2+ to Fe3+ oxidation within the octahedral sites and not directly to the released fluids (e.g. Wang et al., 2012). These particular aspects of the amphibole physical properties have never been studied in detail, thus an accurate description of the deprotonation mechanisms and their relationship with oxidation of multi-valence elements is necessary. In this work we examine a riebeckite from Malawi, chosen among our collection of Fe-rich samples as a representative of the sodic amphiboles group species, common in high-P geological environments. As a starting point to model any modification induced during high temperature treatments, the sample was characterized in details at room-T by means of single crystal XRD refinement, EPMA, IR and Mössbauer spectroscopy. The crystal-chemical formula turned out to be very close to the ideal end-member composition Na2Fe2+3Fe3+2Si8O22(OH)2. HT-IR data collected in-situ on single-crystals showed a significant increase in the absorption coefficient with T, a feature already observed by Della Ventura et al. (2015). Spectra collected on quenched specimens showed an abrupt and complete hydrogen loss in the 550-650°C T range. The behavior of the OH-stretching absorption of riebeckite is different to that of potassic-ferro-richterite (Della Ventura et al., 2015): a decrease in intensity of all bands is observed, and no components to be assigned to the presence of Fe3+ are present in the spectra. This behavior is interpreted in terms of the lack of vibrational coupling (Della Ventura et al., 2007) between facing anionic sites in the structure. The H diffusion mechanism across the structure was addressed by means of isothermal HT-IR experiments, using both doubly-polished single crystals and powders. We observed that the kinetics of deprotonation is strongly influenced by the atmosphere within the heating stage, suggesting a strong control of the oxygen availability during the experiment on the hydrogen diffusion. This issue is presently under detailed investigation.
Crystal chemistry and HT behavior of riebeckite from Malawi: a combined XRD and FTIR study
2015
Abstract
Fe-amphiboles play a major role in enhancing the electrical conductivity of subducted rocks in convergent margins (Reynard et.al., 2011). This issue has been shown to be related to the Fe2+ to Fe3+ oxidation within the octahedral sites and not directly to the released fluids (e.g. Wang et al., 2012). These particular aspects of the amphibole physical properties have never been studied in detail, thus an accurate description of the deprotonation mechanisms and their relationship with oxidation of multi-valence elements is necessary. In this work we examine a riebeckite from Malawi, chosen among our collection of Fe-rich samples as a representative of the sodic amphiboles group species, common in high-P geological environments. As a starting point to model any modification induced during high temperature treatments, the sample was characterized in details at room-T by means of single crystal XRD refinement, EPMA, IR and Mössbauer spectroscopy. The crystal-chemical formula turned out to be very close to the ideal end-member composition Na2Fe2+3Fe3+2Si8O22(OH)2. HT-IR data collected in-situ on single-crystals showed a significant increase in the absorption coefficient with T, a feature already observed by Della Ventura et al. (2015). Spectra collected on quenched specimens showed an abrupt and complete hydrogen loss in the 550-650°C T range. The behavior of the OH-stretching absorption of riebeckite is different to that of potassic-ferro-richterite (Della Ventura et al., 2015): a decrease in intensity of all bands is observed, and no components to be assigned to the presence of Fe3+ are present in the spectra. This behavior is interpreted in terms of the lack of vibrational coupling (Della Ventura et al., 2007) between facing anionic sites in the structure. The H diffusion mechanism across the structure was addressed by means of isothermal HT-IR experiments, using both doubly-polished single crystals and powders. We observed that the kinetics of deprotonation is strongly influenced by the atmosphere within the heating stage, suggesting a strong control of the oxygen availability during the experiment on the hydrogen diffusion. This issue is presently under detailed investigation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
