High-pressure phase transitions of clinoenstatite

Journal Article · Sat Jun 01 00:00:00 EDT 2019 · American Mineralogist

DOI:https://doi.org/10.2138/am-2019-6740· OSTI ID:1542639

Lazarz, John D. ^[1]; Dera, Przemyslaw ^[2]; Hu, Yi ^[3]; Meng, Yue ^[4]; Bina, Craig R. ^[5]; Jacobsen, Steven D. ^[5]

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1. Northwestern Univ., Evanston, IL (United States). Dept. of Earth and Planetary Sciences; Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Univ. of Hawai'i at Manoa, Honolulu, HI (United States). School of Ocean and Earth Science and Technology, Hawaii Inst. of Geophysics and Planetology
3. Univ. of Hawai'i at Manoa, Honolulu, HI (United States). School of Ocean and Earth Science and Technology, Hawaii Inst. of Geophysics and Planetology and Dept. of Geology and Geophysics
4. Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Proton Source, HPCAT
5. Northwestern Univ., Evanston, IL (United States). Dept. of Earth and Planetary Sciences

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 similar to ~ 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 K_T0 = 121(2) GPa and V₀ = 403.9(5) ų (with $$K^{'}_{T0}$$ = 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 in our study, 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.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States); George Washington Univ., Washington, DC (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC); Carnegie/DOE Alliance Center (CDAC); USDOE National Nuclear Security Administration (NNSA), Office of Defense Programs (DP)
• Grant/Contract Number: AC02-06CH11357; NA0003858
• OSTI ID: 1542639
• Alternate ID(s): OSTI ID: 1599885
• Journal Information: American Mineralogist, Vol. 104, Issue 6; ISSN 0003-004X
• Publisher: Mineralogical Society of America
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
MgSiO3
clinoenstatite
enstatite
pyroxene
single-crystal X-ray diffraction