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American Mineralogist

Journal of Earth and Planetary Materials

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

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Volume 104, Issue 6


High-pressure phase transitions of clinoenstatite

John D. Lazarz / Przemyslaw Dera
  • Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A
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/ Yi Hu
  • Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A
  • Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A
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/ Yue Meng / Craig R. Bina
  • Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A
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/ Steven D. Jacobsen
  • Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A
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Published Online: 2019-05-27 | DOI: https://doi.org/10.2138/am-2019-6740


Clinoenstatite (Mg2Si2O6) undergoes a well-known phase transition from a low-pressure form (LPCEN, space group P21/c) to a high-pressure form (HPCEN, space group C2/c) at ~6 GPa. High-pressure structure refinements of HPCEN were carried out based on single-crystal X‑ray diffraction experiments between 9.5 and 35.5 GPa to determine its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metastability. The best-fit isothermal equation of state to our data combined with the five data points between 5.34 and 7.93 GPa from Angel and Hugh-Jones (1994) yields a second-order Birch-Murnaghan equation with KT0 = 121(2) GPa and V0 = 403.9(5) Å3 (with KʹT0 = 4 implied). Further reduction of misfit upon fitting a third-order Birch-Murnaghan equation is not significant at the 90% confidence level. At ~45 GPa, a transition from HPCEN to a P21/c-structured polymorph (HPCEN2) was observed, which is isostructural to the P21/c phase recently observed in diopside (CaMgSi2O6) at 50 GPa (Plonka et al. 2012) and in clinoferrosilite (Fe2Si2O6) at 30–36 GPa (Pakhomova et al. 2017). Observation of HPCEN2 in Mg2Si2O6 completes the third apex of the pyroxene quadrilateral wherein HPCEN2 is found, facilitating a broader view of clinopyroxene crystal chemistry at conditions relevant to metastability in the Earth’s mantle along cold subduction geotherms.

Keywords: MgSiO3; clinoenstatite; enstatite; pyroxene; single-crystal X‑ray diffraction

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About the article

* Present address: Shock and Detonation Physics, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A

Received: 2018-07-19

Accepted: 2019-01-30

Published Online: 2019-05-27

Published in Print: 2019-06-26

Funding This research was supported through grants from U.S. National Science Foundation EAR-1452344 and EAR-1853521 to S.D.J. and EAR-1722969 to P.D. Support was also provided by the Capital/DOE Alliance Center (CDAC). Development of the ATREX software used for data analysis was supported by NSF grant EAR1440005. Portions of this work were performed at HPCAT (Sector 16) of the Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Joshua Townsend for discussions and Sergey Tkachev for gas-loading diamond-anvil cells in the GSECARS gas-loading system, which is supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 1606856.

Citation Information: American Mineralogist, Volume 104, Issue 6, Pages 897–904, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2019-6740.

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