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

Journal Article · · Physics of the Earth and Planetary Interiors
 [1];  [2];  [1]
  1. Princeton Univ., NJ (United States)
  2. Univ. of Hawaii, Honolulu, HI (United States); Univ. of Chicago, IL (United States)

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 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

Research Organization:
Carnegie Inst. of Science, Washington, DC (United States). Geophysical Lab.
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA)
Grant/Contract Number:
NA0002006
OSTI ID:
1338326
Alternate ID(s):
OSTI ID: 1251985
Journal Information:
Physics of the Earth and Planetary Interiors, Vol. 244, Issue C; ISSN 0031-9201
Publisher:
ElsevierCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 38 works
Citation information provided by
Web of Science

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High‐Pressure γ ‐CaMgSi 2 O 6 : Does Penta‐Coordinated Silicon Exist in the Earth's Mantle? journal November 2017
Phase Transitions in Orthoenstatite and Subduction Zone Dynamics: Effects of Water and Transition Metal Ions journal April 2018
An Experimental Investigation of the Relative Strength of the Silica Polymorphs Quartz, Coesite, and Stishovite journal April 2019
High pressure phase transitions of paracelsian BaAl2Si2O8 journal September 2019
Pentacoordinated silicon in the high-pressure modification of datolite, CaBSiO 4 (OH) journal January 2018
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Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts journal November 2018
High-pressure single-crystal X-ray diffraction and synchrotron Mössbauer study of monoclinic ferrosilite journal February 2019
High Pressure Single Crystal Diffraction at PX^2 journal January 2017