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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Putirka, Keith / Swainson, Ian

12 Issues per year


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

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1945-3027
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Volume 100, Issue 8-9

Issues

The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions

Marco Merlini
  • Corresponding author
  • Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133 Milano, Italy
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/ Michael Hanfland
  • ESRF—The European Synchrotron Radiation Facility, 71, avenue des Martyrs, 38000 Grenoble, France
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/ Ashkan Salamat / Sylvain Petitgirard / Harald Müller
  • ESRF—The European Synchrotron Radiation Facility, 71, avenue des Martyrs, 38000 Grenoble, France
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Published Online: 2015-08-12 | DOI: https://doi.org/10.2138/am-2015-5369

Abstract

We simulated the redox decomposition of magnesium-siderite at pressures and temperatures corresponding to the top of the Earth’s D″ layer (135 GPa and 2650 K). It transforms into new phases, with unexpected stoichiometry. We report their crystal structure, based on single-crystal synchrotron radiation diffraction on a multi-grain sample, using a charge-flipping algorithm. Mg2Fe2(C4O13) is monoclinic, a = 9.822(3), b = 3.9023(13), c = 13.154(5) Å, β = 108.02(3)°, V = 479.4(3) Å3 (at 135 GPa). It contains tetrahedrally coordinated carbon units, corner-shared in truncated C4O13 chains. Half of the cations are divalent, and half trivalent. The carbonate coexists with a new iron oxide, Fe13O19, monoclinic, a = 19.233(2), b = 2.5820(13), c = 9.550(11) Å, β = 118.39(3)°, V = 417.2(5) Å3 (at 135 GPa). It has a stoichiometry between hematite, Fe2O3, and magnetite, Fe3O4. The formation of these unquenchable phases indicates, indirectly, the formation of reduced-carbon species, possibly diamond. These structures suggest the ideas that the mineralogy of the lower mantle and D″ region may be more complex than previously estimated. This is especially significant concerning accessory phases of fundamental geochemical significance and their role in ultra-deep iron-carbon redox coupling processes, as well as the iron-oxygen system, which certainly play an important role in the lower mantle mineral phase equilibria.

Keywords: Carbonate; tetrahedrally coordinated carbon; iron oxide; high pressure; single-crystal

About the article

Received: 2015-04-02

Accepted: 2015-05-06

Published Online: 2015-08-12

Published in Print: 2015-08-01


Citation Information: American Mineralogist, Volume 100, Issue 8-9, Pages 2001–2004, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5369.

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© 2015 by Walter de Gruyter Berlin/Boston.

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