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Licensed Unlicensed Requires Authentication Published by De Gruyter November 29, 2019

Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth

Helene Breton , Tetsuya Komabayashi EMAIL logo , Samuel Thompson , Nicola Potts , Christopher McGuire , Sho Suehiro , Simone Anzellini and Yasuo Ohishi
From the journal American Mineralogist

Abstract

Compression and decompression experiments on face-centered cubic (fcc) γʹ-Fe4N to 77 GPa at room temperature were conducted in a diamond-anvil cell with in situ X‑ray diffraction (XRD) to examine its stability under high pressure. In the investigated pressure range, γʹ-Fe4N did not show any structural transitions. However, a peak broadening was observed in the XRD patterns above 60 GPa. The obtained pressure-volume data to 60 GPa were fitted to the third-order Birch-Murnaghan equation of state (EoS), which yielded the following elastic parameters: K0 = 169 (6) GPa, K′ = 4.1 (4), with a fixed V0 = 54.95 Å at 1 bar. A quantitative Schreinemakers’ web was obtained at 15–60 GPa and 300–1600 K by combining the EoS for γʹ-Fe4N with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where γʹ-Fe4N, hexagonal closed-packed (hcp) ε-Fe7N3, double hexagonal closed-packed β-Fe7N3, and hcp Fe phases are stable. From the invariant point, a reaction γʹ-Fe4N = β-Fe7N3 + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of γʹ-Fe4N at 56 GPa and 300 K. Therefore, the γʹ-Fe4N phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that β-Fe7N3 would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe7(C,N)3 found as a mantle-derived diamond inclusion implies that β-Fe7N3 and Fe7C3 may form a continuous solid solution in the mantle deeper than 1000 km depth. Diamond formation may be related to the presence of fluids in the mantle, and dehydration reactions of high-pressure hydrous phase D might have supplied free fluids in the mantle at depths greater than 1000 km. As such, the existence of Fe7(C,N)3 in diamond can be an indicator of water transportation to the deep mantle.


Orcid 0000-0002-1106-1592

† Special collection papers can be found online at http://www.minsocam.org/MSA/AmMin/special-collections.html


Acknowledgments and Funding

We thank K. Litasov and an anonymous reviewer for their helpful comments on the manuscript. Synchrotron XRD measurements were carried out at beamlines BLI15 at the Diamond Light Source (ref number: ee17683-1) and BL10XU at the SPring-8 (ref number: 2018B1464). This work is supported by the Natural Environment Research Council (NERC) (No. NE/M000346/1) and by the European Research Council (ERC) Consolidator Grant to T.K. (No. 647723).

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Received: 2019-03-26
Accepted: 2019-08-25
Published Online: 2019-11-29
Published in Print: 2019-12-18

© 2019 Walter de Gruyter GmbH, Berlin/Boston

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