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Due to its large thermal stability, Al-phase D, the (Al,Fe3+)2SiO6H2 member of the dense hydrous magnesium silicate (DHMS) phase D, may survive along hot subduction geotherms or even at ambient mantle temperatures in the Earth's transition zone and lower mantle, therefore potentially playing a major role as a water reservoir and carrier in the Earth's interior. We have investigated the crystal structure and high-pressure behavior of Fe-bearing Al-phase D with a composition of Al1.53(2)Fe0.22(1) Si0.86(1)O6H3.33(9) by means of single-crystal X-ray diffraction. While the structure of pure Al-phase D (Al2SiO6H2) has space group P63/mcm and consists of equally populated and half-occupied (Al,Si)O6 octahedra, Fe-incorporation in Al-phase D seems to induce partial ordering of the cations over the octahedral sites, resulting in a change of the space group from P63/mcm to P6322 and in well-resolved diffuse scattering streaks observed in X-ray images. The evolution of the unit-cell volume of Fe-bearing Al-phase D between room pressure and 38 GPa, determined by means of synchrotron X-ray diffraction in a diamond anvil cell, is well described by a third-order Birch-Murnaghan equation of state having an isothermal bulk modulus KT0 = 166.3(15) GPa and first pressure derivative K′T0 = 4.46(12). Above 38 GPa, a change in the compression behavior is observed, likely related to the high-to-low spin crossover of octahedrally coordinated Fe3+. The evolution of the unit-cell volume across the spin crossover was modeled using a recently proposed formalism based on crystal-field theory, which shows that the spin crossover region extends from approximately 30 to 65 GPa. Given the absence of abrupt changes in the compression mechanism of Fe-bearing Al-phase D before the spin crossover, we show that the strength of H-bonds and likely their symmetrization do not greatly afect the elastic properties of phase D solid solutions, independently of their compositions. [ABSTRACT FROM AUTHOR]

Stishovite, a rutile-structured polymorph of SiO2, is a main component of subducted basaltic lithologies in the lower mantle. At mid lower-mantle depths, a second-order ferroelastic transition to orthorhombic CaCl2-type (post-stishovite) structure occurs, causing extensive elastic shear softening. Previous studies showed that Al incorporation can decrease the transition pressure, while it is still debated whether H has a similar effect. Here we report the equations of state, structural evolution, and phase transformation of Si0.948Al0.052O1.983H0.018 (Al5) stishovite and Si0.886Al0.114O1.980H0.074 (Al11) post-stishovite samples using diamond-anvil cells in combination with synchrotron X-ray diffraction and Raman spectroscopy. The Al5 sample transformed to the orthorhombic polymorph upon compression to 16 GPa, displaying a drop of ~12% in its bulk modulus across the transformation. The Al11 sample did not undergo any phase transition in the pressure range investigated. Single-crystal structural refinements and Raman spectroscopy measurements on the Al5 sample show that the soft optic mode B1g is decoupled from the tetragonal-to-orthorhombic structural transformation and shows a plateau in the stability field of post-stishovite, between 20 and 30 GPa. This observation indicates that the transformation is not pseudo-proper ferroelastic as in SiO2 stishovite and that existing Landau expansions are likely not applicable to H-rich Al-bearing silica samples. Using the equation of state parameters of orthorhombic Al5 and Al11 and literature data on SiO2 post-stishovite we then discuss the possibility of non-ideal mixing along the SiO2-AlOOH join. [ABSTRACT FROM AUTHOR]

The crystal chemistry of ten well‐characterized bridgmanite single‐crystals with Fe and Al contents ranging from 0 to 0.40 atoms per two‐cation formula units were investigated by single‐crystal X‐ray diffraction. Structural refinements indicate that Fe3+ and Al mainly occupy the Mg and Si sites, respectively, when present in similar proportions. Molar volumes of bridgmanite endmember components were refined using data from this and previous studies and found to decrease in the order Fe3+Fe3+O3 > MgFe3+O2.5 > Fe3+AlO3 > MgAlO2.5 > AlAlO3 > Fe2+SiO3 > MgSiO3. Fe3+AlO3 charge‐coupled substitution leads to an anisotropic increase of B‐O bond distances, resulting in more distorted octahedral B sites and in a more significant increase of the c‐axis with respect to the a‐ and b‐axes. Valence bond calculations indicate that the A site is more compressible than the B site for all bridgmanite samples studied, implying that octahedral tilting and distortion will dominate the bridgmanite compression mechanism. Guided by these crystal chemical observations, bulk moduli of bridgmanite endmember components were estimated using results of previous studies. The volume changes of equilibria controlling the speciation of bridgmanite components were then calculated at conditions relevant to the top of Earth's lower mantle. The proportion of oxygen vacancy components is predicted to decrease with pressure. While the stability of the bridgmanite Fe3+AlO3 component will drive charge disproportionation to produce iron metal at the top of the lower mantle, this appears to be much less favorable by 50 GPa. An increase in the proportion of the Fe3+Fe3+O3 bridgmanite component, however, may favor the formation of iron metal at higher pressures. Plain Language Summary: Fe‐Al‐bearing MgSiO3 bridgmanite is the most abundant mineral in the Earth's lower mantle. Its physical and chemical properties, which are closely related to its crystal structure, play a major role in controlling processes in the Earth's lower mantle. In this study, we synthesized several bridgmanite single crystals with various compositions, which allowed the substitution mechanisms and site occupancies of Fe and Al in the structure to be determined. The changes in volume and structure resulting from the different substitution mechanisms were identified and can be used to explain the compressibility of different bridgmanite endmembers. The effect of pressure on the composition of bridgmanite was then predicted. We found that pressure should favor charge‐coupled substitution where 3+ cations enter both the Mg and Si cation sites. The alternative mechanism, where 3+ cations enter the Si site and oxygen vacancies are created, is not favored with pressure. Also Fe3+ and Al may become disordered onto both the Mg and Si sites as pressure increases. The formation of Fe metal through reduction and oxidation of Fe2+ may be suppressed at greater depths in the lower mantle, therefore, the deep lower mantle may be less reduced than the shallower region. Key Points: The molar volumes of bridgmanite endmember components were determined to be Fe3+Fe3+O3 > MgFe3+O2.5 > Fe3+AlO3 > MgAlO2.5 > AlAlO3 > Fe2+SiO3 > MgSiO3Fe3+ and Al mainly occupy the A and B sites, respectively, at 25 GPa when Fe3+ = Al but disordering may be favored at higher pressuresPressure favors charge‐coupled substitution over oxygen vacancy formation and may suppress Fe metal precipitation [ABSTRACT FROM AUTHOR]

The combination of seismic observations and mineral physics data represents a unique tool to understand the structure and evolution of the deep Earth's interior. However, to date, elasticity data on both compressional (vP) and shear (vS) wave velocities of MgSiO3 bridgmanite are limited to shallow mantle conditions, hampering the resolution of mineral physics models. Here, we report the first single‐crystal measurements of vP and vS of MgSiO3 bridgmanite up to ∼79 GPa using high‐pressure Brillouin scattering and single‐crystal X‐ray diffraction in a diamond anvil cell. At shallow lower mantle pressures, the elastic anisotropy of MgSiO3 bridgmanite was found to be similar, albeit smaller than that of Fe,Al‐bearing bridgmanite of Kurnosov et al. (2017) but differed significantly from that proposed in the recent study of Fu et al. (2019). Using the elastic stiffness coefficients of bridgmanite obtained in this study at different pressures, we calculate the pressure dependence of the adiabatic bulk modulus, KS0 = 257.1(6) GPa, K'S0 = 3.71(4), and of the shear modulus, G0 = 175.6(2) GPa, G'0 = 1.86(1). These elastic parameters are included in a self‐consistent thermodynamic model to calculate seismic wave velocities along a lower mantle adiabat for a primitive upper mantle bulk composition in the FeO‐CaO‐MgO‐SiO2 system, which is currently the most complex system for which sufficient data exist. This preliminary model provides a good match to the vS and vP of 1D seismic models, implying that the composition of the lower mantle may be closer to pyrolite, rather than being more bridgmanite rich. Key Points: We measured both compressional and shear wave velocities of MgSiO3 bridgmanite up to mid‐lower mantle pressure for the first timeThe effect of chemistry on the elastic moduli and elastic anisotropy of bridgmanite at lower mantle pressures is discussedWe compare the seismic velocities for a simplified primitive upper mantle composition with 1D seismic models of Earth's lower mantle [ABSTRACT FROM AUTHOR]