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Phase egg, [AlSiO3(OH)], is an aluminosilicate hydrous mineral that is thermodynamically stable in lithological compositions represented by Al2O3-SiO2-H2O (ASH) ternary, i.e., a simplified ternary for the mineralogy of subducted sediments and continental crustal rocks. High-pressure and high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs show that phase egg is stable up to pressures of 20–30 GPa, which translates to the transition zone to lower mantle depths. Thus, phase egg is a potential candidate for transporting water into the Earth’s mantle transition zone. In this study, we use first-principles simulations based on density functional theory to explore the pressure dependence of crystal structure and how it influences energetics and elasticity. Our results indicate that phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). Such anomalous behavior in the elasticity is related to changes in the hydrogen bonding O-H···O configurations, which we delineate as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is very anisotropic resulting in a maximum anisotropy of compressional wave velocity, AvP ≈ 30% and of shear wave velocity, AvS ≈ 17% at zero pressures. Our results also indicate that the phase egg has one of the fastest bulk sound velocities (vP and vS) compared to other hydrous aluminous phases in the ASH ternary, which include topaz-OH, phase Pi, and d-AlOOH. However, the bulk sound velocity of phase egg is slower than that of stishovite. At depths corresponding to the base of mantle transition zone, phase egg decomposes to a mixture of d-AlOOH and stishovite. The changes in compressional DvP and shear DvS velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm.
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Atomistic Mechanism of Al Substitution Effects on the Ferroelastic Post-stishovite Transition by High-Pressure Single-Crystal X-Ray Diffraction
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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- Award ID(s):
- 2333879
- PAR ID:
- 10588173
- Publisher / Repository:
- American Mineralogist
- Date Published:
- Journal Name:
- American Mineralogist
- ISSN:
- 0003-004X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/am-2023-9257
