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![The stability field of various hydrous minerals under upper mantle, transition zone, and lower mantle conditions. The stability fields of hydrous phase H (present work), phase H-phase δ solid solution (aluminous phase H [Ohira et al., 2014]), and δ-AlOOH [Sano et al., 2007] are shown in this figure. Those of various hydrous minerals, such as serpentine, srp, 10 Å phase, phase A, hydrous wadsleyite, and hydrous ringwoodite, are based on Ohtani [2005]. Those of superhydrous phase B and phase D are based on Ohtani et al. [2004]. The stability field of the solid solution of 44 mol % of phase H ((MgSi)OOH) and 56 mol % of phase δ (AlOOH) estimated by Ohira et al. [2014] is shown as a dashed curve labeled as H0.44δ0.56.](profile/Tatsuya-Sakamaki-2/publication/267570729/figure/fig1/AS:274140636643347@1442371629617/The-stability-field-of-various-hydrous-minerals-under-upper-mantle-transition-zone-and.png)
The stability field of various hydrous minerals under upper mantle, transition zone, and lower mantle conditions. The stability fields of hydrous phase H (present work), phase H-phase δ solid solution (aluminous phase H [Ohira et al., 2014]), and δ-AlOOH [Sano et al., 2007] are shown in this figure. Those of various hydrous minerals, such as serpentine, srp, 10 Å phase, phase A, hydrous wadsleyite, and hydrous ringwoodite, are based on Ohtani [2005]. Those of superhydrous phase B and phase D are based on Ohtani et al. [2004]. The stability field of the solid solution of 44 mol % of phase H ((MgSi)OOH) and 56 mol % of phase δ (AlOOH) estimated by Ohira et al. [2014] is shown as a dashed curve labeled as H0.44δ0.56.
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We report the stability field of a new high-pressure hydrous phase, phase H MgSiO4H2, and its implications for water transport into the deep lower mantle. We observed the existence of hydrous phase H at pressures around 50 GPa and this phase was stable up to 60 GPa. Our results, together with those of previous works, indicate that pure phase H MgSi...
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... a solid solution between phase H and phase δ-AlOOH, i.e., aluminous phase H, (MgSiAl 2 )O 4 H 2 , can exist stably under the transition zone and in lower mantle conditions, as reported by Ohira et al. [2014]. Figure 3 summarizes the stability fields of phase H, together with various hydrous phases reported previously. The stability field of the solid solution of 44 mol % of phase H ((MgSi)OOH) and 56 mol % phase δ (AlOOH) estimated by Ohira et al. [2014] is also shown in this figure. ...
Citations
... Extensive past studies focused on various dense hydrous magnesium silicates, such as phase A * These authors contributed equally to this work. † [17][18][19][20][21][22][23][24][25][26][27], as potential deep-Earth water carriers [28]. However, these dense hydrous magnesium silicates are not fully compatible with the stability requirements in the geologically relevant environments, as some dissociate into an assemblage of nominally anhydrous phases plus water at the lower-mantle pressure and temperature conditions (<1500 km) [16], while all of them possess lower density compared to the preliminary reference Earth model (PREM) data. ...
... Meanwhile, the β-FeSiO 4 H 2 phase spans a wider pressure-temperature stability field, from 32 to 61 GPa and up to 1450 K. The threshold temperature for the thermal stability of the FeSiO 4 H 2 phases first increases with rising pressure, then decreases steeply when pressure exceeds 40 GPa, which is similar to the behavior of phase H MgSiO 4 H 2 under pressure [24][25][26]. ...
... Geological studies suggest a nearly dry bridgmanitedominated environment in vast lower-mantle regions [41]. This scenario stems from a lack of known hydrous minerals that can be stabilized in the deep lower-mantle geotherm conditions [24][25][26][27]. There is, however, experimental evidence showing that Al could enhance the thermal stability of hydrous magnesium silicates [76,77]. ...
Hydrous minerals hold the key to unlocking the enduring mystery of the water cycle deep inside the Earth. Tremendous efforts have been devoted to identifying geologically viable minerals meeting stringent pressure-temperature-density stability requirements for descent into deep Earth, and such pursuits remain active. Here, we identify two hydrous iron silicates, α- and β−FeSiO4H2, formed by a reaction of Earth-abundant FeSiO3 and H2O and stabilized at the pressure-temperature conditions in cold subducting slabs. These phases have a sufficiently high density for a stable descent into the Earth's lower mantle, and then decompose to release water after reaching equilibrium with the mantle geotherm. Moreover, Mg(Fe)SiO4H2 solutions are found to be more stable than the pure substances and can serve as effective carriers to transport substantial amounts of water to lower-mantle regions via the cold subduction zones. These findings establish a viable and robust material basis for the deep-Earth water cycle, with major implications for elucidation of many prominent geological processes.
... The stability of dense hydrous magnesium silicates (DHMS) at high-pressure and high-temperature conditions has been extensively studied for more than 20 years, as these phases can potentially carry and recycle water from Earth's surface to its deep interior (e.g., Frost 2006;Nishi et al. 2014;Ohtani et al. 2014). The crystal structure of DHMS generally consists of hexagonal closest-packed (hcp) layers of O atoms with Mg and Si occupying interstitial octahedral and tetrahedral sites, respectively. ...
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 KT0′ = 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 affect the elastic properties of phase D solid solutions, independently of their compositions.
... Moreover, recent studies by Nisr et al. (2020) and Lin et al. (2020Lin et al. ( , 2022 indicate that stishovite and post-stishovite phases in the oceanic crust can accommodate several wt% of water in the MTZ and lower mantle. Accordingly, water transportation into the MTZ, lower mantle, and the core-mantle boundary (CMB) has been widely proposed (Lin et al., 2020(Lin et al., , 2022Nisr et al., 2020;Ohtani et al., 2014;Walter et al., 2015). ...
... (b) Dry solidus and liquidus of the lower mantle by Andrault et al. (2011), wet solidus by Nomura et al. (2014) and saturated solidus (MgO−Al 2 O 3 −SiO 2 −H 2 O system) by Walter et al. (2015) have been adopted. The stability of dense hydrous magnesium silicates phases in the lower mantle is from Nishi et al. (2014) and Ohtani et al. (2014). Collected solidus and liquidus from the literature are also shown in Figure S1. ...
... In-situ maximum allowed water content of each rock phase is calculated as a function of pressure and temperature from a pre-computed look-up table. The stability of serpentine (all DHMS phases are regarded here as the serpentine phase at high pressures for visualization purposes) is taken from Schmidt and Poli (1998), Iwamori (2004) and Ohtani et al. (2014). The water content of hydrated ultramafic rocks is downscaled to 30% of its maximum to account for heterogeneous hydration at the trench ( Figure 1) (Chen & Faccenda, 2019;J. ...
The distribution of water within the Earth's mantle has significant implications for the Earth's dynamics and evolution. Recent mineral physics experiments indicate that dense hydrous magnesium silicates can contain large amounts of water stable up to 60 GPa or even beyond along slab geotherms. Here we perform petrological‐thermomechanical numerical simulations of water transportation by deep slab subduction and related magmatism in the mid‐mantle. Key parameters including those defining the slab thermal parameter and the water storage capacity in the oceanic lithosphere and surrounding mantle are explored. The results show two major dehydration events of ultramafic rocks at around 150 and 750 km by dehydration of serpentine at 600°C and superhydrous phase B in the entrained wet upper mantle, respectively. Large amounts of water, ∼1.5 wt% at least locally, are carried down to the mantle transition zone and lower mantle. We estimate an upper limit of slab water flux into the mid‐mantle of 0.1–0.28 × 10¹² kg/yr, which is ∼13%–37% of the input water from the serpentinized mantle. Moreover, a substantial fraction of the water released by the slab is absorbed by the entrained mantle and overlying mid‐mantle portions, such that ∼30%–70% of the water injected at the trench could be delivered to the lower mantle. The deepest magmatism is observed at ∼1,500 km in case of phase H breakdown (MgO‐SiO2‐H2O system), coinciding with the depth of strong seismic attenuation. Overall, these simulations suggest that up to 0.2 ocean mass per billion years could be transported down to the mid‐mantle and beyond.
... 2 of 17 . The stability of phase H at high PT has been studied by theoretical predictions (Tsuchiya, 2013;Tsuchiya & Umemoto, 2019) and experiments Ohtani et al., 2014;Walter et al., 2015). Previous experiments suggested that phase H has an orthorhombic structure with space group Pnnm (Bindi et al., 2014). ...
... Previous experiments suggested that phase H has an orthorhombic structure with space group Pnnm (Bindi et al., 2014). Phase H will dehydrate into bridgmanite at the depth of 1,300-1,700 km in a cold slab (Nishi et al., 2018;Ohtani et al., 2014;Tsuchiya, 2013;Tsuchiya & Umemoto, 2019;Walter et al., 2015). Coincidentally, seismic studies (e.g., Courtier & Revenaugh, 2008;Frost et al., 2018;Kaneshima, 2019;Schumacher & Thomas, 2016;Waszek et al., 2018) found seismic discontinuities globally with variable depths (1,300-1,700 km), lateral extent, orientation, and seismic polarity. ...
... In a cold slab, Phase H, which may be transformed from phase D around the depth of 1,000 km (Tsuchiya, 2013), will undergo the dehydration reaction (Phase H = Bridgmanite + H2O ) at the depth of ∼1,300-1,700 km (52-69 GPa) (Nishi et al., 2018;Ohtani et al., 2014;Tsuchiya, 2013;Tsuchiya & Umemoto, 2019;Walter et al., 2015). The form of H 2 O released by the dehydration of phase H is still debatable. ...
Phase H (MgSiO4H2), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1,300–1,700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1,500 km on VP, VS, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs.
... The stability field of the lower-pressure phase is shown in Fig. 1. A novel hydrous magnesium silicate phase has also been studied with experimental and computational methods [40][41][42] and is predicted to be stable below ∼ 1500 K over a pressure range of approximately 42-60 GPa. ...
Super-Earths and sub-Neptunes are the most common planet types in our galaxy. A subset of these planets is predicted to be water worlds, bodies that are rich in water and poor in hydrogen gas. The interior structures of water worlds have been assumed to consist of water surrounding a rocky mantle and iron core. In small planets, water and rock form distinct layers with limited incorporation of water into silicate phases, but these materials may interact differently during the growth and evolution of water worlds due to greater interior pressures and temperatures. Here, we use density functional molecular dynamics (DFT-MD) simulations to study the miscibility and interactions of enstatite (MgSiO3), a major end-member silicate phase, and water (H2O) at extreme conditions in water world interiors. We explore pressures ranging from 30 to 120 GPa and temperatures from 500 to 8000 K. Our results demonstrate that enstatite and water are miscible in all proportions if the temperature exceeds the melting point of MgSiO3. Furthermore, we performed smoothed particle hydrodynamics simulations to demonstrate that the conditions necessary for rock-water miscibility are reached during giant impacts between water-rich bodies of 0.7–4.7 Earth masses. Our simulations lead to water worlds that include a mixed layer of rock and water.
... Recent studies indicate that aluminum substitution into DHMSs increases the thermodynamic stability of these phases [7,[12][13][14][15][16][17][18], and that Al-bearing DHMSs may host more water than their magnesium endmember counterparts [18,19]. Additionally, Al-bearing phase D is a likely precursor to the solid solution formed by phase H [MgAlO 2 (OH) 2 ] and δ-(Al,Fe)OOH-a solid solution with P-T stability that extends to the core-mantle boundary [13,14,20,21]. ...
Using first-principles calculations, this study evaluates the structure, equation of state, and elasticity of three compositions of phase D up to 75 GPa: (1) the magnesium endmember [MgSi2O4(OH)2], (2) the aluminum endmember [Al2SiO4(OH)2], and (3) phase D with 50% Al- substitution [AlMg0.5Si1.5O4(OH)2]. We find that the Mg-endmember undergoes hydrogen-bond symmetrization and that this symmetrization is linked to a 22% increase in the bulk modulus of phase D, in agreement with previous studies. Al2SiO4(OH)2 also undergoes hydrogen-bond symmetrization, but the concomitant increase in bulk modulus is only 13%—a a significant departure from the 22% increase of the Mg-end member. Additionally, Al-endmember phase D is denser (2%–6%), less compressible (6%–25%), and has faster compressional (6%–12%) and shear velocities (12%–15%) relative to its Mg-endmember counterpart. Finally, we investigated the properties of phase D with 50% Al-substitution [AlMg0.5Si1.5O4(OH)2], and found that the hydrogen-bond symmetrization, equation of state parameters, and elastic constants of this tie-line composition cannot be accurately modeled by interpolating the properties of the Mg- and Al-endmembers.
... However, it has recently been proposed that polymorphs of MgSiO 4 H 2 may be important carriers of water in the lower mantle at pressures exceeding 45 GPa (Tsuchiya 2013;Bindi et al. 2014;Caracas 2017, 2020). One such polymorph, phase H, topologically equivalent to δ-AlOOH, was experimentally found to exist at pressures of 35-60 GPa and temperatures below ~1500 K (Ohtani et al. 2014). In agreement with the experiments, a computational study predicts that phase H exists up to 60 GPa at 1000 K, decomposing into bridgmanite + ice VII at higher pressures (Tsuchiya and Umemoto 2019). ...
Using particle swarm optimization with density functional theory, we identify the positions of hydrogen in a hypothetical Mg-end-member of phase egg (MgSiO4H2) and predict the most stable crystal structures with MgSiO4H2 stoichiometry at pressures between 0 and 300 GPa. The particle swarm optimization method consistently and systematically identifies phase H as the energetically most stable structure in the pressure range 10–300 GPa at 0 K. Phase Mg-egg has a slightly higher energy compared to phase H at all relevant pressures, such that the energy difference nearly plateaus at high pressures; however, the combined effects of temperature and chemical substitutions may decrease or even reverse the energy difference between the two structures. We find a new MgSiO4H2 phase with the P43212 space group that has topological similarities to phase Mg-egg and is energetically preferred to phase H at 0–10 GPa and 0 K. We compute the free energies for phase Mg-egg, phase P43212, and phase H at 0–30 GPa within the quasi-harmonic approximation and find that the effect of temperature is relatively small. At 1800 K, the stability field of phase P43212 relative to the other polymorphs increases to 0–14 GPa, while pure phase Mg-egg remains energetically unfavorable at all pressures. Simulated X-ray diffraction patterns and Raman spectra are provided for the three phases. Additionally, the crystallographic information for two metastable polymorphs with the P1 space group is provided. Our results have implications for the deep hydrogen cycle in that we identify two novel potential carrier phases for hydrogen in the mantles of terrestrial planets and assess their stability relative to phase H. We determine that further experimental and computational investigation of an extended compositional space remains necessary to establish the most stable dense hydrated silicate phases.
... Goethite transforms to ε-FeOOH (isostructural with δ-AlOOH) at pressures above 5 GPa at 200 °C (Gleason et al. 2008), which then transforms into pyrite-type FeOOH at pressures between 60-90 GPa and temperatures exceeding 1227 °C (Nishi et al. 2017). Experimental results show that δ-AlOOH, ε-FeOOH, and PhH (MgSiH 2 O 4 ) may form solid solutions in the Earth's deep lower mantle (Sano et al. 2008;Ohira et al. 2014;Ohtani et al. 2014;Nishi et al. 2017;Xu et al. 2019a). However, the stability of δ-AlOOH and ε-FeOOH in the Mg,Si-bearing systems under transition zone conditions has not been investigated. ...
Dense hydrous magnesium silicates (DHMSs) are considered important water carriers in the deep Earth. Due to the significant effect of Fe on the stability of DHMSs, Fe-bearing Phase D (PhD) deserves much attention. However, few experiments have been conducted to determine the stability of PhD in different bulk compositions. In this study, we provide experimental constraints for the stability of PhD in the AlOOH-FeOOH-Mg1.11Si1.89O6H2.22 system between 18 and 25 GPa at 1000–1600 °C, corresponding to the P-T conditions of the mantle transition zone and uppermost lower mantle.
Fe3+-bearing PhD was synthesized from the FeOOH-Mg1.11Si1.89O6H2.22 binary system with two different Fe3+ contents. The resultant Al,Fe3+-bearing compositions are close to analog specimens of the fully oxidized mid-ocean ridge basalt (MORB) and pyrolite in the AlOOH-FeOOH-Mg1.11Si1.89O6H2.22 ternary system. The substitution mechanism of Fe is shown to be dependent on pressure, and Fe3+ occupies both Mg and Si sites in PhD at pressures below 21 GPa. In contrast, Fe3+ only occupies Si site at pressures exceeding 21 GPa. The presence of Fe3+ results in a slight reduction in the thermal stability field of PhD in the FeOOH-Mg1.11Si1.89O6H2.22 system in comparison to Mg-bearing, Fe-free PhD. In contrast, Al,Fe3+-bearing PhD is more stable than Mg-bearing PhD in both MORB and pyrolite compositions. In this regard, Al,Fe3+-bearing PhD could act as a long-term water reservoir during subduction processes to the deep mantle.
... Despite dehydration phenomena, a slab can still retain in its most stable water-bearing minerals, i.e. the -phase (Duan et al., 2018), phase- ( -Nishi et al., 2014;Ohtani et al., 2014) which is stable up to (i.e. ), and the pyrite type (Hu et al., 2016;Nishi et al., 2017;Yuan et al., 2018). Depending on the thermal evolution of a slab, these phases can reach the (Mashino, 2016;Yuan et al., 2019). ...
A Combined Study on Earth's Deep Water Cycle using Numerical Modelling and Laboratory Experiments. The distinctive feature of Earth’s surface compared to other known planets is the abundance of liquid water. This water was delivered during the accretion stage of the planet by rocky asteroids, with minor contributions from comets and protosolar nebular gas. Experimental evidence shows that water can be incorporated into many of the minerals that make up Earth’s interior. When water is hosted in the crystalline structures, it alters the physical properties of minerals, thereby enhancing deformation processes. Therefore, water-bearing rocks are less dense and weaker compared to their dry counterparts. Geophysical observations and natural samples reveal that water is indeed present in Earth’s mantle, mostly concentrated in the mantle transition zone. The region is bounded by two seismic discontinuities, at 410 km and 660 km depth, and is characterized by the presence of two minerals with high water solubility: wadsleyite and ringwoodite. Water is carried into Earth’s interior by the subduction of oceanic lithosphere, i.e. slabs. This water-delivery mechanism is also known as the ‘deep water cycle,’ and might represent the key element for the onset of plate tectonics on Earth. One essential tool to explore the role of water in plate tectonics is numerical modelling. With this technique, it is possible to reproduce many physical phenomena occurring on Earth by self-consistently simulating mantle convection. This is achieved by solving the governing equations of mass, momentum and energy conservation, also known as Stokes equations. The complexity of these equations requires the use of approximation methods, like Finite Difference (FD), to solve the derivatives over time and space. The feedback loop between mineral physics constraints, numerical modelling and geophysical observations represents the best strategy to unravel Earth’s interior. However, despite the efforts of geoscientists, many questions regarding the deep Earth water cycle remain so far unanswered. This thesis focuses on the hydration state of the MTZ, with three aims addressing different aspects of the topic: (1) provide mineral physics measurements on the effect of water on ringwoodite thermal conductivity; (2) produce a model featuring an Earth-like mobile lid while minimizing the effects of numerical parameters; and (3) analyse the parameters that allow for the stagnation of a slab in the MTZ, which may lead to the water enrichment in this region In project (1), hydrous ringwoodite crystals were synthesized with multi-anvil experiments and characterized by X-ray diffraction, electron-microprobe analysis, and infrared spectroscopy. The samples were loaded into a diamond anvil cell to perform measurements at the high-pressure conditions of the MTZ. The thermal conductivity of ringwoodite, Λ(Rw) was measured with the time-domain thermo-reflectance method. It was found that the presence of 1.73 wt% water reduces Λ(Rw) by 40%. From this analysis, it was possible to derive a parameterized equation to extrapolate Λ(Rw) as a function of pressure and water content. With this tool, the large-scale thermal evolution of a slab was studied. The calculations were performed by assuming a slab stagnating in the MTZ, then being progressively heated by the warm ambient mantle. A 1D FD numerical code was designed to solve the heat diffusion equation, and the derived equation for Λ(Rw) was included into the physical model of the slab. The results reveal that hydrous ringwoodite hinders the heating of the slab, thus promoting the survival of water-bearing minerals. In project (2), a global-scale model to reproduce self-consistently plate-like behaviour was designed. The models were computed with StagYY in a 2D spherical annulus geometry. The tectonic regime of a planet is controlled by the yield strength of the lithosphere τy. Four main tectonic regimes can be identified in nature: micro-plate dripping, plate-like, episodic resurfacing, and stagnant lid. It was found that the tectonic regime in the models is heavily influenced by the grid resolution used for the discretization. The modelled lithosphere is weakened by reducing the horizontal grid spacing ∆w, while it becomes stronger when reducing the vertical grid spacing ∆r. These effects are numerical in nature, and are the consequence of the interpolation of the Stokes equations. It was found that the best results are achieved by accurately resolving the lithosphere, i.e. ∆w ≤40 km and ∆r ≤15 km. In project (3), the interactions between the cold descending slab and the 660 km discontinuity were analysed. The models were computed with StagYY in a 2D spherical annulus geometry. From the models it can be inferred that the stagnation of a slab in the MTZ is controlled by three parameters: (i) the density jump ∆ρ, which enhances the slab pull, and affects the latent heat absorbed by the post-spinel reaction; (ii) the negative Clapeyron slope Υ, which causes a downward deflection of the phase transition; and (iii) the viscosity jump ∆η, which decelerates the descent of the slab.
... Therefore, over the past 20 years, many hydrous minerals, such as dense hydrous magnesium silicates [5], have been synthesized at high-pressure (P) and high-temperature (T ) conditions and investigated as potential candidates for hydrogen transport to the lower mantle. However, most of these minerals decompose at P < 60 GPa, where phase H breaks down to MgSiO 3 bridgmanite and a fluid component [6][7][8]. ...
δ−AlOOH is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a P21nm to a Pnnm structure, the nature and origin of hydrogen disorder, the symmetrization of the O-H⋯O hydrogen bond and their interplay. We perform a series of density functional theory-based simulations in combination with high-pressure nuclear magnetic resonance (NMR) experiments on δ−AlOOH up to 40 GPa with the goal to better characterize the hydrogen potential and therefore the nature of hydrogen disorder. Simulations predict a phase transition in agreement with our NMR experiments at 10−11GPa and hydrogen bond symmetrization at 14.7GPa. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our NMR data.
