High-pressure phase transitions of clinoenstatite

Abstract Clinoenstatite (Mg2Si2O6) undergoes a well-known phase transition from a low-pressure form (LPCEN, space group P21/c) to a high-pressure form (HPCEN, space group C2/c) at ~6 GPa. High-pressure structure refinements of HPCEN were carried out based on single-crystal X‑ray diffraction experiments between 9.5 and 35.5 GPa to determine its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metastability. The best-fit isothermal equation of state to our data combined with the five data points between 5.34 and 7.93 GPa from Angel and Hugh-Jones (1994) yields a second-order Birch-Murnaghan equation with KT0 = 121(2) GPa and V0 = 403.9(5) Å3 (with KʹT0 = 4 implied). Further reduction of misfit upon fitting a third-order Birch-Murnaghan equation is not significant at the 90% confidence level. At ~45 GPa, a transition from HPCEN to a P21/c-structured polymorph (HPCEN2) was observed, which is isostructural to the P21/c phase recently observed in diopside (CaMgSi2O6) at 50 GPa (Plonka et al. 2012) and in clinoferrosilite (Fe2Si2O6) at 30–36 GPa (Pakhomova et al. 2017). Observation of HPCEN2 in Mg2Si2O6 completes the third apex of the pyroxene quadrilateral wherein HPCEN2 is found, facilitating a broader view of clinopyroxene crystal chemistry at conditions relevant to metastability in the Earth’s mantle along cold subduction geotherms.

[1]  C. Bina,et al.  High‐Pressure γ‐CaMgSi2O6: Does Penta‐Coordinated Silicon Exist in the Earth's Mantle? , 2017 .

[2]  L. Dubrovinsky,et al.  A new high-pressure phase transition in clinoferrosilite: In situ single-crystal X-ray diffraction study , 2017 .

[3]  P. Dera,et al.  Single-crystal diffraction and Raman spectroscopy of hedenbergite up to 33 GPa , 2015, Physics and Chemistry of Minerals.

[4]  D. Rubie,et al.  Why cold slabs stagnate in the transition zone , 2015 .

[5]  J. Hunen,et al.  The effect of metastable pyroxene on the slab dynamics , 2014 .

[6]  T. Duffy,et al.  Phase transitions in orthopyroxene (En90) to 49GPa from single-crystal X-ray diffraction , 2014 .

[7]  R. Angel,et al.  EosFit7c and a Fortran module (library) for equation of state calculations , 2014 .

[8]  Timo Heister,et al.  BurnMan: A lower mantle mineral physics toolkit , 2013 .

[9]  S. Clark,et al.  High pressure single-crystal micro X-ray diffraction analysis with GSE_ADA/RSV software , 2013 .

[10]  C. Bina.  Mineralogy: Garnet goes hungry , 2013 .

[11]  D. Rubie,et al.  Stagnation of subducting slabs in the transition zone due to slow diffusion in majoritic garnet , 2013 .

[12]  A. Plonka,et al.  β‐diopside, a new ultrahigh‐pressure polymorph of CaMgSi2O6with six‐coordinated silicon , 2012 .

[13]  N. Bolfan-Casanova,et al.  Water storage capacity in olivine and pyroxene to 14 GPa: Implications for the water content of the Earth's upper mantle and nature of seismic discontinuities , 2012 .

[14]  D. Neuville,et al.  Elasticity of diopside to 8 GPa and 1073 K and implications for the upper mantle , 2010 .

[15]  R. Hemley,et al.  Effect of H2O on upper mantle phase transitions in MgSiO3: Is the depth of the seismic X-discontinuity an indicator of mantle water content? , 2010 .

[16]  Y. Nakajima,et al.  Orthoenstatite/clinoenstatite phase transformation in MgSiO3 at high-pressure and high-temperature determined by in situ X-ray diffraction: Implications for nature of the X discontinuity , 2009 .

[17]  R. Wentzcovitch,et al.  Low-pressure clino- to high-pressure clinoenstatite phase transition: A phonon-related mechanism , 2009 .

[18]  T. Kikegawa,et al.  Survival of pyropic garnet in subducting plates , 2008 .

[19]  V. Prakapenka,et al.  The COMPRES/GSECARS gas-loading system for diamond anvil cells at the Advanced Photon Source , 2008 .

[20]  Takeyuki Uchida,et al.  In situ measurements of sound velocities and densities across the orthopyroxene → high-pressure clinopyroxene transition in MgSiO3 at high pressure , 2004 .

[21]  J. Woodhouse,et al.  The nature of the Lehmann discontinuity from its seismological Clapeyron slopes , 2004 .

[22]  Chung-Cherng Lin.  Pressure-induced polymorphism in enstatite (MgSiO3) at room temperature: clinoenstatite and orthoenstatite , 2004 .

[23]  S. Stein,et al.  Implications of slab mineralogy for subduction dynamics , 2001 .

[24]  R. Stalder,et al.  The Mg(Fe)SiO3 orthoenstatite-clinoenstatite transitions at high pressures and temperatures determined by Raman-spectroscopy on quenched samples , 2001 .

[25]  M. Hanfland,et al.  A P21/c-C2/c high-pressure phase transition in Ca0.5Mg1.5Si2O6 clinopyroxene , 2001 .

[26]  Mauro Prencipe,et al.  High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6) , 2000 .

[27]  N. Tomioka,et al.  In situ X-ray diffraction study of enstatite up to 12 GPa and 1473 K and equations of state , 1999 .

[28]  B. Reynard,et al.  The effect of iron on the P21lc to C2/c transition in (Mg,Fe)SiO3 clinopyroxenes , 1999 .

[29]  R. Angel,et al.  Effect of Ca2+ and Fe2+ on the equation of state of MgSiO3 orthopyroxene , 1997 .

[30]  A. Kutoglu,et al.  Single-crystal compression and crystal structure of clinopyroxene up to 10 GPa , 1997 .

[31]  Burke,et al.  Generalized Gradient Approximation Made Simple. , 1996, Physical review letters.

[32]  Blöchl,et al.  Projector augmented-wave method. , 1994, Physical review. B, Condensed matter.

[33]  R. Angel,et al.  Equations of state and thermodynamic properties of enstatite pyroxenes , 1994 .

[34]  Hafner,et al.  Ab initio molecular dynamics for open-shell transition metals. , 1993, Physical review. B, Condensed matter.

[35]  R. Angel,et al.  Stability of high-density clinoenstatite at upper-mantle pressures , 1992, Nature.

[36]  T. Jordan,et al.  Mantle layering from ScS reverberations: 3. The upper mantle , 1991 .

[37]  T. Gasparik,et al.  Reversals of the orthoenstatite‐clinoenstatite transition at high pressures and high temperatures , 1990 .

[38]  T. Gasparik.  Transformation of enstatite — diopside — jadeite pyroxenes to garnet , 1989 .

[39]  Nobuo Morimoto,et al.  Nomenclature of Pyroxenes , 1988, Mineralogical Magazine.

[40]  Peter M. Bell,et al.  Calibration of the ruby pressure gauge to 800 kbar under quasi‐hydrostatic conditions , 1986 .

[41]  N. Kuroda,et al.  Clinoenstatite in boninites from the Bonin Islands, Japan , 1980, Nature.

[42]  S. Akimoto,et al.  Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures , 1977 .

[43]  A. E. Ringwood,et al.  Phase transformations in descending plates and implications for mantle dynamics , 1976 .

[44]  P. R. Bevington,et al.  Data Reduction and Error Analysis for the Physical Sciences , 1969 .

[45]  H. H. Hess,et al.  Pyroxenes in the Crystallization of Basaltic Magma , 1951, The Journal of Geology.

[46]  A. Schmidt.  Thermal Equation of State , 2019, Technical Thermodynamics for Engineers.

[47]  H. Ohfuji,et al.  Slow Si-Al interdiffusion in garnet and stagnation of subducting slabs , 2013 .

[48]  G. Sheldrick.  A short history of SHELX. , 2008, Acta crystallographica. Section A, Foundations of crystallography.

[49]  F. Nestola,et al.  High pressure behavior, transformation and crystal structure of synthetic iron-free pigeonite , 2004 .

[50]  F. Seifert,et al.  Metastability of enstatite in deep subducting lithosphere , 1994, Nature.