Metastable high-pressure transformations of orthoferrosilite Fs82

Przemyslaw Dera, Gregory J. Finkelstein, Thomas S. Duffy, Robert T Downs, Yue Meng, Vitali Prakapenka, Sergey Tkachev

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Abstract

High-pressure single-crystal X-ray diffraction experiments with natural ferrosilite Fs82 (Fe2+0.82Mg0.16Al0.01Ca0.01)(Si0.99Al0.01)O3 orthopyroxene (opx) reveal that at ambient temperature the sample does not transform to the clinopyroxene (cpx) structure, as reported earlier for a synthetic Fs100 end-member (Hugh-Jones et al., 1996), but instead undergoes a series of two polymorphic transitions, first above 10.1(1)GPa, to the monoclinic P21/c phase β-opx (distinctly different from both P21/c and C2/c cpx), also observed in natural enstatite (Zhang et al., 2012), and then, above 12.3(1)GPa to a high-pressure orthorhombic Pbca phase γ-opx, predicted for MgSiO3 by atomistic simulations (Jahn, 2008). The structures of phases α, β and γ have been determined from the single-crystal data at pressures of 2.3(1), 11.1(1), and 14.6(1)GPa, respectively. The two new high-pressure transitions, very similar in their character to the P21/c-C2/c transformation of cpx, make opx approximately as dense as cpx above 12.3(1)GPa and significantly change the elastic anisotropy of the crystal, with the [100] direction becoming almost twice as stiff as in the ambient α-opx phase. Both transformations involve mainly tetrahedral rotation, are reversible and are not expected to leave microstructural evidence that could be used as a geobarometric proxy. The high Fe2+ content in Fs82 shifts the α-β transition to slightly lower pressure, compared to MgSiO3, and has a very dramatic effect on reducing the (meta) stability range of the β-phase.

Original languageEnglish (US)
Pages (from-to)15-21
Number of pages7
JournalPhysics of the Earth and Planetary Interiors
Volume221
DOIs
StatePublished - Aug 2013

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orthopyroxene
clinopyroxene
enstatite
crystal
elastic anisotropy
single crystals
metastable state
ambient temperature
low pressure
shift
diffraction
anisotropy
transform
crystals
X-ray diffraction
x rays
simulation
experiment
temperature

Keywords

  • Enstatite
  • Ferrosilite
  • High-pressure phase transition
  • Pyroxenes
  • Synchrotron single-crystal X-ray diffraction
  • Upper mantle

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science
  • Physics and Astronomy (miscellaneous)
  • Astronomy and Astrophysics

Cite this

Metastable high-pressure transformations of orthoferrosilite Fs82. / Dera, Przemyslaw; Finkelstein, Gregory J.; Duffy, Thomas S.; Downs, Robert T; Meng, Yue; Prakapenka, Vitali; Tkachev, Sergey.

In: Physics of the Earth and Planetary Interiors, Vol. 221, 08.2013, p. 15-21.

Research output: Contribution to journalArticle

Dera, Przemyslaw ; Finkelstein, Gregory J. ; Duffy, Thomas S. ; Downs, Robert T ; Meng, Yue ; Prakapenka, Vitali ; Tkachev, Sergey. / Metastable high-pressure transformations of orthoferrosilite Fs82. In: Physics of the Earth and Planetary Interiors. 2013 ; Vol. 221. pp. 15-21.
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AU - Meng, Yue

AU - Prakapenka, Vitali

AU - Tkachev, Sergey

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