Compressional behavior of omphacite to 47 GPa
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Omphacite is an important mineral component of eclogite. Single-crystal synchrotron X-ray diffraction data on natural (Ca, Na) (Mg, Fe, Al)Si2O6 omphacite have been collected at the Advanced Photon Source beamlines 13-BM-C and 13-ID-D up to 47 GPa at ambient temperature. Unit cell parameter and crystal structure refinements were carried out to constrain the isothermal equation of state and compression mechanism. The third-order Birch–Murnaghan equation of state (BM3) fit of all data gives V 0 = 423.9(3) Å3, K T0 = 116(2) GPa and K T0′ = 4.3(2). These elastic parameters are consistent with the general trend of the diopside–jadeite join. The eight-coordinated polyhedra (M2 and M21) are the most compressible and contribute to majority of the unit cell compression, while the SiO4 tetrahedra (Si1 and Si2) behave as rigid structural units and are the most incompressible. Axial compressibilities are determined by fitting linearized BM3 equation of state to pressure dependences of unit cell parameters. Throughout the investigated pressure range, the b-axis is more compressible than the c-axis. The axial compressibility of the a-axis is the largest among the three axes at 0 GPa, yet it quickly drops to the smallest at pressures above 5 GPa, which is explained by the rotation of the stiffest major compression axis toward the a-axis with the increase in pressure.
KeywordsPyroxene High pressure Single-crystal diffraction Synchrotron Mantle Subduction
The project was supported by the National Science Foundation Division of Earth Sciences Geophysics Grant No.1344942. Development of ATREX IDL software is supported under National Science Foundation Grant No. 1440005. This work was performed at GeoSoilEnviroCARS (Sector 13), Partnership for Extreme Crystallography program (PX^2), Advanced Photon Source (APS) and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and Department of Energy—Geosciences (DE-FG02-94ER14466). PX^2 program is supported by COMPRES under NSF Cooperative Agreement EAR 11-57758. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-C02-6CH11357. Use of the COMPRES-GSECARS gas-loading system was supported by COMPRES under NSF Cooperative Agreement EAR 11-57758 and by GSECARS through NSF grant EAR-1128799 and DOE grant DE-FG02-94ER14466. We would like to thank Prof. R. T. Downs at the University of Arizona for kindly providing the samples from RRUFF collections, Dr. R. J. Angel at the University of Padova for the helpful discussions during the 49th International School of Crystallography, the two anonymous reviewers for the very positive, thorough, and constructive comments and Dr. Milan Rieder for handling the manuscript.
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