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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

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1945-3027
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Volume 100, Issue 5-6

Issues

Fayalite oxidation processes in Obsidian Cliffs rhyolite flow, Oregon

Audrey M. Martin
  • Corresponding author
  • NASA Johnson Space Center, Mailcode KT, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
  • Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44118, U.S.A.
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  • Other articles by this author:
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/ Etienne Médard
  • Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS-IRD, OPGC, 5 rue Kessler, 63038 Clermont-Ferrand, France
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/ Bertrand Devouard
  • Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS-IRD, OPGC, 5 rue Kessler, 63038 Clermont-Ferrand, France
  • CEREGE, AMU-CNRS-IRD (UM34), Technopôle Arbois-Méditerranée, BP 80, 13545 Aix-en-Provence, France
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/ Lindsay P. Keller / Kevin Righter / Jean-Luc Devidal
  • Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS-IRD, OPGC, 5 rue Kessler, 63038 Clermont-Ferrand, France
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Published Online: 2015-05-12 | DOI: https://doi.org/10.2138/am-2015-5042

Abstract

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+1.60.8(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+2.321.16(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.

Keywords: Olivine; fayalite; laihunite; oxyfayalite; rhyolite; lithophysae; oxidation

About the article

Received: 2014-05-08

Accepted: 2014-10-29

Published Online: 2015-05-12

Published in Print: 2015-05-01


Citation Information: American Mineralogist, Volume 100, Issue 5-6, Pages 1153–1164, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5042.

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© 2015 by Walter de Gruyter Berlin/Boston.

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