The unique speciation of iron in calc-alkaline obsidians
| Autor: | Georges Calas, Laurence Galoisy |
|---|---|
| Přispěvatelé: | Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), Muséum national d'Histoire naturelle (MNHN)-Institut de recherche pour le développement [IRD] : UR206-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS) |
| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Předmět: |
Basalt
Valence (chemistry) Felsic 010504 meteorology & atmospheric sciences Silicic Geology 010502 geochemistry & geophysics 01 natural sciences law.invention chemistry.chemical_compound Crystallography chemistry Geochemistry and Petrology law [SDU]Sciences of the Universe [physics] Rhyolite Obsidian RhyoliteGlass Magnetite Color UV-visible spectroscopy EPR Volcanic glasses Electron paramagnetic resonance 0105 earth and related environmental sciences Visible spectrum Magnetite |
| Zdroj: | Chemical Geology Chemical Geology, 2021, 559, pp.119925-. ⟨10.1016/j.chemgeo.2020.119925⟩ |
| ISSN: | 0009-2541 |
| DOI: | 10.1016/j.chemgeo.2020.119925⟩ |
| Popis: | Obsidians found in silicic domes and rhyolitic tuffs are an ideal system to understand the formation conditions of calc-alkaline felsic magmas. The Optical Absorption and Electron Paramagnetic Resonance (EPR) spectroscopic data presented in this study demonstrate that Fe-speciation in obsidians is different from that observed in other geological glasses (e.g., basaltic glasses) or in synthetic Fe-bearing silicate glasses. The presence of unusual symmetric Fe2+ octahedral sites explains the characteristic black to brown color of obsidian glasses, much distinct from that of synthetic Fe-bearing glasses. The Fe3+ sites partly occur in superparamagnetic domains, as evidenced by EPR data. The dramatic intensification of the optical absorption in the visible range indicates the presence of Fe-rich domains with Fe2+-Fe3+ and/or Fe2+-Ti4+ Inter-Valence Charge Transfer (IVCT) processes. These data are consistent with the presence of Fe-rich, mixed valence clusters, located outside the glass structure. The largest of these domains contribute to the black color of obsidians because they cause a background absorption extending throughout the visible spectrum as observed in magnetite. This unusual Fe-speciation reflects the last events of the magmatic history during emplacement and cooling, indicating that obsidians cannot be considered as a quenched witness of the initial magma. |
| Databáze: | OpenAIRE |
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