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Article Dans Une Revue Chemical Geology Année : 2021

The unique speciation of iron in calc-alkaline obsidians

Résumé

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.
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hal-03493054 , version 1 (07-11-2022)

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Paternité - Pas d'utilisation commerciale

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Laurence Galoisy, Georges Calas. The unique speciation of iron in calc-alkaline obsidians. Chemical Geology, 2021, 559, pp.119925 -. ⟨10.1016/j.chemgeo.2020.119925⟩. ⟨hal-03493054⟩
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