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Title: Phase transitions in orthopyroxene (En 90) to 49GPa from single-crystal X-ray diffraction

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on ~Mg 0.9Fe 0.1SiO 3 (En 90) 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-popxmore » structures show a progressive 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
 [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:
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
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
36 MATERIALS SCIENCE; Orthopyroxene; Phase transition; Crystallography; High pressure; Single-crystal X-ray diffraction
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1251985