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American Mineralogist

Journal of Earth and Planetary Materials

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


IMPACT FACTOR 2018: 2.631

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0003-004X
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Volume 100, Issue 5-6

Issues

Optical properties of siderite (FeCO3) across the spin transition: Crossover to iron-rich carbonates in the lower mantle

Sergey S. Lobanov
  • Corresponding author
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
  • V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, Koptyuga pr. 3, Novosibirsk 630090, Russia
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  • Other articles by this author:
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/ Alexander F. Goncharov
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
  • Center for High Energy Matter in Extreme Environments and Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 350 Shushanghu Road, Hefei, Anhui 230031, China
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/ Konstantin D. Litasov
  • V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, Koptyuga pr. 3, Novosibirsk 630090, Russia
  • Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russia
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Published Online: 2015-05-12 | DOI: https://doi.org/10.2138/am-2015-5053

Abstract

Upper mantle carbonates are thought to be iron poor and magnesium rich. However, at lower mantle conditions spin-pairing transitions in iron-bearing phases may trigger iron redistribution between the minerals. Here, using visible and near infrared absorption measurements, we examine the siderite crystal field up to 65 GPa. Optical spectrum of siderite at 1 bar has an absorption band at 10 325 cm-1 corresponding to the crystal field splitting energy (10Dq) of ferrous iron in an octahedral field. This band intensifies and blue-shifts (86 cm-1/GPa) with pressure, but disappears abruptly at 44 GPa signaling the spin transition. Simultaneously, a new absorption band centered at 15 629 cm-1 (88 cm-1/GPa) appears in the spectrum. Tanabe-Sugano diagram analysis allowed assigning the observed absorption bands to 5T2g5Eg and 1A1g1T1g electronic transitions in high- and low-spin siderite, respectively. Similarly, we evaluate the crystal field splitting energy of low-spin siderite 10Dq = 17 600 cm-1 (45 GPa), as well as the Racah parameters B = 747 cm-1 and C = 3080 cm-1. We find that the crystal field stabilization energy (CFSE) of ferrous iron in low-spin siderite (45 700 cm-1 at 45 GPa) is an order of magnitude higher than that in the high-spin phase (4130 cm-1 at 1 bar). From the derived CFSE values we estimate the iron-partitioning coefficient for the carbonate-perovskite system and show that lowspin carbonates are iron rich and magnesium poor. We also show that the color of siderite is governed by the 1Ag1T1g absorption band and the Fe-O charge transfer.

Keywords: Crystal field; high pressure; crystal chemistry; Tanabe-Sugano diagram; iron partitioning

About the article

Received: 2014-05-13

Accepted: 2014-11-08

Published Online: 2015-05-12

Published in Print: 2015-05-01


Citation Information: American Mineralogist, Volume 100, Issue 5-6, Pages 1059–1064, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5053.

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© 2015 by Walter de Gruyter Berlin/Boston.

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