Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSi[O.sub.3] akimotoite

MgSi[O.sub.3] akimotoite is stable relative to majoritegarnet 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 [H.sub.2]O) 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.sub.1] = 20(3) x [10.sup.-9] [K.sup.-2] and [a.sub.0] = 17(2) x [10.sup.-6] [K.sup.-1], with an average of [α.sub.0] = 27.1(6) x [10.sup.-6] [K.sup.-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.sub.0] = 263.7(2) [Å.sup.3], [K.sub.T0] = 217(3) GPa (K fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: [α.sub.a] = 8.2(3) and [α.sub.c] = 10.68(9) ([10.sup.-6][K.sup.-1]); [β.sub.a] = 11.4(3) and [β.sub.c] = 15.9(3) ([10.sup.-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 x [10.sup.-5] and +1.7 x [10.sup.-5] [K.sup.-1], indicating both weak hydrogen bonds (O-H ... O) and strong OH bonding due to long O ... O distances. Keywords Akimotoite * Crystal structure * Thermal expansion * Compressibility * Anisotropy * FTIR
Introduction Akimotoite (ilmenite-type MgSi[O.sub.3]) is stable at pressures of 18- 25 GPa and up to about 2,000 K in the MgSi[O.sub.3] system, with wadsleyite plus stishovite stable at lower pressures, [...]