Discovery of a hexagonal ultradense hydrous phase in (Fe,Al)OOH

Zhang, Li; Yuan, Hongsheng; Meng, Yue; Mao, Ho‐kwang

doi:10.1073/pnas.1720510115

Cited by 20 publications

(15 citation statements)

References 32 publications

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“…We may expect the new iron superoxide in pyrite structure to accommodate and form solid solutions with other divalent and trivalent cations such as Mg 2+ and Al 3+ (ref. 48 ) and with anions, such as sulfur and halogens. We may further expect that the peroxide structure is only one example of the possible compositions in the deep Earth, and that additional structure types with different valence and spin, such as the recently discovered hexagonal phase 48 , will emerge.…”

Section: Discussionmentioning

confidence: 99%

Altered chemistry of oxygen and iron under deep Earth conditions

Liu

et al. 2019

Nat Commun

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A drastically altered chemistry was recently discovered in the Fe-O-H system under deep Earth conditions, involving the formation of iron superoxide (FeO2Hx with x = 0 to 1), but the puzzling crystal chemistry of this system at high pressures is largely unknown. Here we present evidence that despite the high O/Fe ratio in FeO2Hx, iron remains in the ferrous, spin-paired and non-magnetic state at 60–133 GPa, while the presence of hydrogen has minimal effects on the valence of iron. The reduced iron is accompanied by oxidized oxygen due to oxygen-oxygen interactions. The valence of oxygen is not –2 as in all other major mantle minerals, instead it varies around –1. This result indicates that like iron, oxygen may have multiple valence states in our planet’s interior. Our study suggests a possible change in the chemical paradigm of how oxygen, iron, and hydrogen behave under deep Earth conditions.

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Section: Discussionmentioning

confidence: 99%

Altered chemistry of oxygen and iron under deep Earth conditions

Liu

et al. 2019

Nat Commun

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“…The major components of the lower mantle are made of nominally anhydrous minerals, which contain no more than 1 weight percent of water [1,2]. However, recent discoveries of deep hydrous phase (DHP) including δ-AlOOH [3][4][5], phase H [6], HH-phase [7], and pyrite-type FeO 2 Hx [8,9], which were synthesized at conditions of cold mantle geotherm from natural minerals like diaspore (α-AlOOH) and goethite (α-FeOOH), provide possible mechanisms to transport a significant amount of water to the bottom of the mantle. The potential presence of hydrogen is likely to contribute a variety of seismological features observed at Earth's lower mantle.…”

Section: Introductionmentioning

confidence: 99%

Experimental Evidence for Partially Dehydrogenated ε-FeOOH

et al. 2019

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Hydrogen in hydrous minerals becomes highly mobile as it approaches the geotherm of the lower mantle. Its diffusion and transportation behaviors under high pressure are important in order to understand the crystallographic properties of hydrous minerals. However, they are difficult to characterize due to the limit of weak X-ray signals from hydrogen. In this study, we measured the volume changes of hydrous ε-FeOOH under quasi-hydrostatic and non-hydrostatic conditions. Its equation of states was set as the cap line to compare with ε-FeOOH reheated and decompression from the higher pressure pyrite-FeO 2 Hx phase with 0 < x < 1. We found the volumes of those re-crystallized ε-FeOOH were generally 2.2% to 2.7% lower than fully hydrogenated ε-FeOOH. Our observations indicated that ε-FeOOH transformed from pyrite-FeO 2 Hx may inherit the hydrogen loss that occurred at the pyrite-phase. Hydrous minerals with partial dehydrogenation like ε-FeOOHx may bring it to a shallower depth (e.g., < 1700 km) of the lower mantle.

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“…The incorporation of other elements (e.g., Al, Si, Mg) may also affect melting of P phase. A recent experimental study found that (Fe,Al)O 2 H (the solid solution phase of AlO 2 H and FeO 2 H) is stable at 107–136 GPa and 2,400 K (Zhang et al, ). Duan et al (), on the other hand, reported that AlOOH dehydrates at 2,500 K and 142 GPa.…”

Section: Resultsmentioning

confidence: 99%

First‐Principles Study of FeO₂H_x Solid and Melt System at High Pressures: Implications for Ultralow‐Velocity Zones

et al. 2019

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Pyrite-type FeO 2 H x (P phase) has recently been suggested as a possible alternative to explain ultralow-velocity zones due to its low seismic velocity and high density. Here we report the results on the congruent melting temperature and melt properties of P phase at high pressures from first-principles molecular dynamics simulations. The results show that P phase would likely be melted near the core-mantle boundary. Liquid FeO 2 H x has smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO 2 H x could account for the observed seismic anomaly of ultralow-velocity zones. However, to maintain the liquid FeO 2 H x within the ultralow-velocity zones against compaction requires special physical conditions, such as relatively high viscosity of the solid matrix and/or vigorous convection of the overlying mantle.Plain Language Summary Ultralow-velocity zones (ULVZs) are 5-40-km-thick patches lying above Earth's core-mantle boundary. They are characterized with anomalously low seismic velocities compared with the ambient mantle and may contain important clues on the thermochemical evolution of the Earth. A recent experimental study argued that ULVZs may be caused by the accumulation of pyrite-type FeO 2 H x (P phase) at the bottom of the mantle. Here for the first time, we systematically study the thermoelastic properties of both FeO 2 H x solid and liquid phases. We find that P phase is likely melted near the core-mantle boundary and thus cannot be the source of ULVZs. Furthermore, in order for the molten product of P phase to cause ULVZs, the dense and nearly inviscid melts must be dynamically stable and confined within the ULVZs, which requires that the mantle is highly viscous and/or convects vigorously.

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Discovery of a hexagonal ultradense hydrous phase in (Fe,Al)OOH

Cited by 20 publications

References 32 publications

Altered chemistry of oxygen and iron under deep Earth conditions

Altered chemistry of oxygen and iron under deep Earth conditions

Experimental Evidence for Partially Dehydrogenated ε-FeOOH

First‐Principles Study of FeO₂H_x Solid and Melt System at High Pressures: Implications for Ultralow‐Velocity Zones

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Discovery of a hexagonal ultradense hydrous phase in (Fe,Al)OOH

Cited by 20 publications

References 32 publications

Altered chemistry of oxygen and iron under deep Earth conditions

Altered chemistry of oxygen and iron under deep Earth conditions

Experimental Evidence for Partially Dehydrogenated ε-FeOOH

First‐Principles Study of FeO2Hx Solid and Melt System at High Pressures: Implications for Ultralow‐Velocity Zones

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Product

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About

First‐Principles Study of FeO₂H_x Solid and Melt System at High Pressures: Implications for Ultralow‐Velocity Zones