TY - JOUR
T1 - Phase transitions in orthopyroxene (En90) to 49GPa from single-crystal X-ray diffraction
AU - Finkelstein, Gregory J.
AU - Dera, Przemyslaw K.
AU - Duffy, Thomas S.
N1 - Funding Information:
We thank the staffs of GSECARS and HPCAT for experimental assistance. Robert Downs (Arizona) provided the procrystal bonding analysis software and Nancy Ross (Virginia Tech) assisted with its use. We thank Jeremy Delaney (Rutgers) for assistance with microprobe measurements. This work was supported by the National Science Foundation . Portions of this work were performed at GSECARS (Sector 13) and HPCAT (Sector 16), Advanced Photon Source, Argonne National Laboratory. GSECARS and HPCAT are supported by the Department of Energy and National Science Foundation . Use of the gas loading system was supported by GSECARS and COMPRES.
Publisher Copyright:
© 2014 Elsevier B.V.
PY - 2015/7/1
Y1 - 2015/7/1
N2 - 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 48.5GPa. The sample was compressed in a diamond anvil cell with a neon pressure-transmitting 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 29.9GPa and 40.3GPa. 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-, 4+1-, 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 4+1-coordinated Si in distorted square pyramids. β-popx retains the mixed 4- and 4+1-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.
AB - 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 48.5GPa. The sample was compressed in a diamond anvil cell with a neon pressure-transmitting 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 29.9GPa and 40.3GPa. 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-, 4+1-, 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 4+1-coordinated Si in distorted square pyramids. β-popx retains the mixed 4- and 4+1-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.
KW - Crystallography
KW - High pressure
KW - Orthopyroxene
KW - Phase transition
KW - Single-crystal X-ray diffraction
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U2 - 10.1016/j.pepi.2014.10.009
DO - 10.1016/j.pepi.2014.10.009
M3 - Article
AN - SCOPUS:84930206212
SN - 0031-9201
VL - 244
SP - 78
EP - 86
JO - Physics of the Earth and Planetary Interiors
JF - Physics of the Earth and Planetary Interiors
ER -