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Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSiO3 akimotoite |
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| Authors: | Yu Ye Joseph R Smyth Steven D Jacobsen Wendy R Panero David A Brown Tomoo Katsura Yun-Yuan Chang Joshua P Townsend Przemyslaw Dera Sergey Tkachev Cayman Unterborn Zhenxian Liu Céline Goujon |
| Institution: | 1. Department of Physics, University of Colorado, Boulder, CO, 80309, USA 9. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA 2. Department of Geological Sciences, University of Colorado, Boulder, CO, 80309, USA 3. Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL, 60208, USA 4. School of Earth Sciences, Ohio State University, Columbus, OH, 43210, USA 5. Bayerisches Geoinstitut, Universit?t Bayreuth, 95440, Bayreuth, Germany 6. Center for Advanced Radiation Sources, University of Chicago, Argonne National Laboratory, Argonne, IL, 60439, USA 7. Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, 20015, USA 8. Institut Néel, CNRS and Université Joseph Fourier, BP 166, 38042, Grenoble Cedex 9, France |
| Abstract: | MgSiO3 akimotoite is stable relative to majorite-garnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg–akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt H2O) was synthesized at 22 GPa and 1,500 °C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250 °C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153–839 K) of a 1 = 20(3) × 10?9 K?2 and a 0 = 17(2) × 10?6 K?1, with an average of α 0 = 27.1(6) × 10?6 K?1. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch–Murnaghan equation of state (BM2 EoS) fitting yields: V 0 = 263.7(2) Å3, K T0 = 217(3) GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: α a = 8.2(3) and α c = 10.68(9) (10?6 K?1); β a = 11.4(3) and β c = 15.9(3) (10?4 GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15–300 K and 0–28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between ?1.02 × 10?5 and +1.7 × 10?5 K?1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances. |
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