Dynamics of Fe oxidation in riebeckite: A model for amphiboles

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Marco E. Ciriotti
Messaggi: 24615
Iscritto il: ven 25 giu, 2004 11:31
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Dynamics of Fe oxidation in riebeckite: A model for amphiboles

Messaggio da Marco E. Ciriotti » gio 05 lug, 2018 15:25

Referenza:
▪ Della Ventura, G., Mihailova, B., Susta, U., Cestelli Guidi, M., Marcelli, A., Schlüter, J., Oberti, R. (2018): The dynamics of Fe oxidation in riebeckite: A model for amphiboles. American Mineralogist, 103, 1103–1111.

Abstract:
In this work, we investigate the oxidation behavior of a nearly end-member riebeckite, ideally Na2(Fe32+Fe23+)Si8O22(OH)2, by using vibrational FTIR and Raman spectroscopies. Combining these results with previous studies performed on the same sample by single-crystal structure refinement and Mössbauer spectroscopy, we conclude that iron oxidation in riebeckite is a multi-step process. (1) In the ~523 K < T < 623 K temperature range, the O-H bond lengthens and both the electrons and the hydrogen cations delocalize. Raman analysis shows that this step is reversible upon cooling to room temperature. (2) In the 623 K < T < 723 K range, the kinetic energy increases so that the electrons can be ejected from the crystal; beyond 723 K an irreversible oxidation of Fe occurs that couples with irreversible changes in the SiO4 double-chains leading to a contraction of the unit-cell volume, i.e., to structural changes detectable at the long-range scale. (3) Beyond 823 K, the irreversible oxidation is completed and H+ ions are forced to leave the crystal bulk. Because of this multi-step process, the onset of the deprotonation process is detected at ~700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mössbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). Also, Raman scattering shows that the release of H+ from the crystal surface starts ~100 K before the complete deprotonation of the crystal bulk is witnessed by FTIR absorption. Hence, the oxidation of Fe starts at the crystal surface and induces electron and H+ migration from the crystal interior to the rim and thus subsequent oxidation through the crystal bulk. No deprotonation is observed by FTIR either in powders embedded in KBr or in crystals heated in N2 atmosphere, implying that the release of H+ needs surficial (atmospheric) oxygen to form H2O molecules. Fe2+ → Fe3+ oxidation produces a flux of electrons throughout the crystal matrix, which generates electrical conductivity across the amphibole. An important implication of this work, which might have interesting applications in material science, is that iron oxidation in riebeckite (and possibly in other Fe-rich silicates) is reversible in a given range of temperature. Also, this work shows that complex processes cannot be fully understood or even monitored accurately without using a proper combination of independent techniques.

NdR: Giancarlo ha presentato un contributo su questo argomento al recente Convegno all'Accademio delle Scienze di Torino: viewtopic.php?f=5&t=14492&p=247685&hili ... no#p247685
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

VINCENZO MILETTO
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Iscritto il: mer 31 gen, 2007 16:44
Località: Cavour - Torino

Re: Dynamics of Fe oxidation in riebeckite: A model for amphiboles

Messaggio da VINCENZO MILETTO » gio 05 lug, 2018 20:24

Alcune fotografie delle slides presentate al convegno di Torino.
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Marco E. Ciriotti
Messaggi: 24615
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Re: Dynamics of Fe oxidation in riebeckite: A model for amphiboles

Messaggio da Marco E. Ciriotti » gio 05 lug, 2018 20:47

Bravo Vinci.
Proprio quelle.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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