| Autor: |
Martin, Audrey M., Médard, Etienne, Devouard, Bertrand, Keller, Lindsay P., Righter, Kevin, Devidal, Jean-Luc |
| Předmět: |
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| Zdroj: |
American Mineralogist; May2015, Vol. 100 Issue 5/6, p1153-1164, 12p |
| Abstrakt: |
This study investigates the oxidation of fayalite Fe22+SiO4 that is present in lithophysae from a rhyolite flow (Obsidian Cliffs, Oregon). Textural, chemical, and structural analyses of the successive oxidation zones are used to constrain: (1) the oxidation processes of olivine, and (2) the role of temperature, chemical diffusion, and meteoric infiltration. Petrologic analyses and thermodynamic modeling show that the rhyolite flow emplaced at 800-950 °C. Fayalite-bearing lithophysae formed only in the core of the lava flow. Variations in the gas composition inside the lithophysae induced the oxidation of fayalite to a laihunite-1M zone Fe12+Fe23+□1(SiO4)2. This zone is made of nano-lamellae of amorphous silica SiO2 and laihunite-3M Fe2+1.6Fe3+1.6□0.8(SiO4)2+ hematite Fe2O3. It probably formed by a nucleation and growth process in the fayalite fractures and defects and at fayalite crystal edges. The laihunite-1M zone then oxidized into an 'oxyfayalite' zone with the composition Fe2+0.52Fe3+2.32□1.16(SiO4)2. This second oxidation zone is made of lamellae of amorphous silica SiO2 and hematite Fe2O3, with a possible small amount of ferrosilite Fe2+SiO3. A third and outer zone, composed exclusively of hematite, is also present. The successive oxidation zones suggest that there may be a mineral in the olivine group with higher Fe3+ content than laihunite-1M. The transformation of laihunite-1M to this 'oxyfayalite' phase could occur by a reaction such as This would imply that Fe3+ can also be incorporated in the M1 site of olivine. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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