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Abstract The influence of Al substitution on the elastic properties of stishovite and its transition to post-stishovite is of great importance for interpreting the seismic wave velocities of subducted mid-ocean ridge basalt (MORB) within the mantle transition zone and the lower mantle. However, atomistic mechanisms of Al substitution effects on the transition and its associated elasticity remain debated. Here synchrotron single-crystal X-ray diffraction measurements have been performed at room temperature on Al1.3-SiO2 (1.3 mol% Al in the chemical formula of Si0.965(3)Al0.041(1)O2H0.017(4)) and Al2.1-SiO2 (2.1 mol% Al in Si0.948(2)Al0.064(1)O2H 0.018(3)) crystals in diamond anvil cells with Boehler-Almax designed anvils up to 38.0 GPa and 28.5 GPa, respectively. Refinements of the diffraction patterns show that a transformation from stishovite (space group P42/mnm; No. 136) to CaCl2-typed post-stishovite (space group Pnnm; No. 58) is accompanied by splitting of O coordinates. The Al substitution in stishovite results in a faster decrease in the O coordinate, softer apical (Si,Al)-O bonds, and a softer and less distorted (Si,Al)O6 octahedron under compression. This leads to reduced adiabatic bulk modulus (KS), shear modulus (G), shear wave velocity (VS), and compressional wave velocity (VP) in the stishovite phase, explaining seismic wave perturbations in the mantle transition zone. Together with Raman data, Landau theory modeling shows that Al substitution increases the order parameter and excess free energy, stabilizing the post-stishovite phase at lower pressures. Correlation between elasticity and octahedral distortion index (D) reveals that at certain D, the Al substitution reduces KS, G, VS, and VP of the stishovite phase while increasing G, VS, and VP of the post-stishovite phase. Importantly, the maximum shear reduction is slightly enhanced at D = 0.00620(9) at the transition point. Our results help explain the seismically observed small-scale VS anomalies beneath subduction regions in the shallow lower mantle where Al,H-bearing stishovite undergoes the post-stishovite transition.
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Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction
Abstract The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase.
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- Award ID(s):
- 1916941
- PAR ID:
- 10405518
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 108
- Issue:
- 1
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 110 to 119
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/am-2022-8458
