TY  - JOUR
AU  - Li, Xinyang
AU  - Speziale, Sergio
AU  - Glazyrin, Konstantin
AU  - Wilke, Franziska D. H.
AU  - Liermann, Hanns-Peter
AU  - Koch-Mueller, Monika
TI  - Synthesis, structure refinement and single-crystal elasticity of Al-bearing superhydrous phase B
JO  - American mineralogist
VL  - 107
IS  - 5
SN  - 0003-004X
CY  - Alexandria, Va.
PB  - GeoScienceWorld
M1  - PUBDB-2021-03999
SP  - 885 - 895
PY  - 2021
AB  - Dense hydrous magnesium silicates (DHMSs) with large water content and wide stability fields are a potential H2O reservoir in the deep Earth. Al-bearing superhydrous phase B (shy-B) with a wider stability field than the Al-free counterpart can play an important role in understanding H<sub>2</sub>O transport in the Earth’s transition zone and topmost lower mantle. In this study, a nominally Al-free and two different Al-bearing shy-B with 0.47(2) and 1.35(4) Al atoms per formula unit (pfu), were synthesized using a rotating multi-anvil press. The single-crystal structures were investigated by X-ray diffraction (XRD) complemented by Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Single-crystal XRD shows that the cell parameters decrease with increasing Al-content. By combining X-ray diffraction and spectroscopy results, we conclude that the Al-poor shy-B crystallizes in the Pnn2 space group with hydrogen in two different general positions. Based on the results of the single crystal X-ray diffraction refinements combined with FTIR spectroscopy, three substitutions mechanisms are proposed: 2 Al<sup>3+</sup> = Mg<sup>2+</sup> + Si<sup>4+</sup>; Mg<sup>2+</sup> ☐<sup>Mg2+</sup> + 2H<sup>+</sup> (☐<sup>Mg2+</sup> means vacancy in Mg site); ; Si<sup>4+</sup> = Al<sup>3+</sup> + H<sup>+</sup>. Thus, in addition to the two general H positions, hydrogen is incorporated into the hydrous mineral via point defects. The elastic stiffness coefficients were measured for the Al-shy-B with 1.35 pfu Al by Brillouin scattering (BS). Al-bearing shy-B shows lower C<sub>11</sub>, higher C<sub>22</sub> and similar C<sub>33</sub> when compared to Al-free shy-B. The elastic anisotropy of Al-bearing shy-B is also higher than that of the Al-free composition. Such different elastic properties are due to the effect of lattice contraction as a whole and the specific chemical substitution mechanism that affect bonds strength. Al-bearing shy-B with lower velocity, higher anisotropy and wider thermodynamic stability can help to understand the low velocity zone and high anisotropy region in the subducted slab located in Tonga.
LB  - PUB:(DE-HGF)16
UR  - <Go to ISI:>//WOS:000790666400010
DO  - DOI:10.2138/am-2022-7989
UR  - https://bib-pubdb1.desy.de/record/465687
ER  -