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Title: Phase transitions in orthopyroxene (En90) to 49GPa from single-crystal X-ray diffraction

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on ~Mg0.9Fe0.1SiO3 (En90) orthopyroxene crystals at room temperature to a maximum pressure of 48.5 GPa. The sample was compressed in a diamond anvil cell with a neon pressure medium and a gold pressure calibrant. In addition to the previously described orthopyroxene to β-opx transition (designated HPCEN2 in previous studies), we observe two further phase transitions at 29.9 GPa and 40.3 GPa. However, we do not observe the γ-opx phase recently described in an Fe-rich orthopyroxene composition. The structures of both of the new phases were solved in space group Pca21. While their Mg-O layers remain pyroxene-like, their Si-O layers transform in a stepwise fashion to akimotoite-like sheets, with sites in 4-, 5-, or 6-fold coordination, depending on the specific structure and layer. Due to the increased Si-O coordination number, we designate the new structures α- and β-post-orthopyroxene (α-popx and β-popx). α-popx has one Si-O layer that is entirely tetrahedral, and one layer that contains both tetrahedra and 5-coordinated Si in distorted square pyramids. β-popx retains the mixed 4- and 5-coordinated Si layer found in α-popx, while the other Si layer adopts fully octahedral coordination. The α- and β-popx structures show a progressivemore » transformation towards the arrangement of Si layers found in akimotoite, a potentially important phase in the earth’s transition zone. Metastable transformations in pyroxenes are of interest for understanding possible metastability in geological environments such as subducting slabs and meteorite impacts« less
Authors:
 [1] ;  [2] ;  [1]
  1. Princeton Univ., NJ (United States)
  2. Univ. of Hawaii, Honolulu, HI (United States); Univ. of Chicago, IL (United States)
Publication Date:
Grant/Contract Number:
NA0002006
Type:
Accepted Manuscript
Journal Name:
Physics of the Earth and Planetary Interiors
Additional Journal Information:
Journal Volume: 244; Journal Issue: C; Journal ID: ISSN 0031-9201
Publisher:
Elsevier
Research Org:
Carnegie Inst. of Washington, Argonne, IL (United States). Geophysical Lab.
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; Orthopyroxene; Phase transition; Crystallography; High pressure; Single-crystal X-ray diffraction
OSTI Identifier:
1338326
Alternate Identifier(s):
OSTI ID: 1251985