The pyrite-type high-pressure form of FeOOH

Abstract

Water transported into Earth's interior by subduction strongly influences dynamics such as volcanism and plate tectonics. Several recent studies have reported hydrous minerals to be stable at pressure and temperature conditions representative of Earth's deep interior, implying that surface water may be transported as far as the core-mantle boundary. However, the hydrous mineral goethite, α-FeOOH, was recently reported to decompose under the conditions of the middle region of the lower mantle to form FeO and release H, suggesting the upward migration of hydrogen and large fluctuations in the oxygen distribution within the Earth system. Here we report the stability of FeOOH phases at the pressure and temperature conditions of the deep lower mantle, based on first-principles calculations and in situ X-ray diffraction experiments. In contrast to previous work suggesting the dehydrogenation of FeOOH into FeO in the middle of the lower mantle, we report the formation of a new FeOOH phase with the pyrite-type framework of FeO octahedra, which is much denser than the surrounding mantle and is stable at the conditions of the base of the mantle. Pyrite-type FeOOH may stabilize as a solid solution with other hydrous minerals in deeply subducted slabs, and could form in subducted banded iron formations. Deep-seated pyrite-type FeOOH eventually dissociates into FeO and releases HO when subducted slabs are heated at the base of the mantle. This process may cause the incorporation of hydrogen into the outer core by the formation of iron hydride, FeH, in the reducing environment of the core-mantle boundary.

Elasticity of ε‐FeOOH: Seismic implications for Earth's lower mantle

“…The addition of a Hubbard U term is critical to accurately describe the stability, electronic, magnetic, and elastic properties of iron‐bearing systems, as is the selection of an appropriate U value [e.g., Jang et al ., ]. In the case of GGA + U calculations, Hubbard U values were determined by an internally consistent method [ Cococcioni and de Gironcoli , ; Tsuchiya et al ., ; Nishi et al ., ], and a secondary set of calculations were performed with a fixed value of U = 5 to allow direct comparison with literature values [e.g., Gleason et al ., 2008]. All values reported herein reflect self‐consistent U value calculations unless otherwise indicated.…”

“…And since only small vol % of FeOOH are needed to mimic the shear wave velocity reductions of LLSVPs, it is plausible that iron‐enriched solid solutions from the FeOOH–AlOOH–MgSiH 2 O 4 system contribute to the observed seismic properties of large low‐shear velocity provinces in the Earth's lower mantle. These intermediate compositions would bear a seismic signature similar to that of LLSVPs, and the thermodynamic (i.e., P‐T ) stability of the phases are likely enhanced by the addition of aluminum, which has been experimentally shown to substantially increase the stability of phase H [ Nishi et al ., ; Ohira et al ., ; Panero and Caracas , ]. Additionally, the horizontally polarized shear wave anisotropy of ε‐FeOOH implies that iron‐rich solid solutions from the FeOOH–AlOOH–MgSiH 2 O 4 system, exhibiting LPO in which the c axis is vertically aligned due to the strong shear stresses at the CMB, could contribute to the seismically observed S H > S V polarization at the base of the mantle [ Karato , ].…”

Ultrahigh‐Pressure Phase Transitions in FeS2 and FeO2: Implications for Super‐Earths' Deep Interior

“…This surprising discovery intrigues us to explore stable structures of materials under extreme conditions. However, later investigations have confirmed the formation of the pyrite-type FeOOH from α-FeOOH at high P-T conditions without any loss of hydrogen (Nishi et al, 2017). In particular, theoretical calculations predict that FeO 2 exhibits extreme pressure stability in contrast to other well-known iron oxides such as FeO, Fe 3 O 4 , and Fe 2 O 3 under high pressure (Weerasinghe et al, 2015).…”

Chemical Reactions Between Fe and H2O up to Megabar Pressures and Implications for Water Storage in the Earth's Mantle and Core

“…The iron peroxide FeO 2 , with a pyrite structure (space group Pa ), was found to be stable at high pressures (Weerasinghe et al, ) and was recently synthesized through the reaction between hematite (Fe 2 O 3 ) and oxygen (O 2 ) (Hu et al, ). Further studies suggest that goethite (α‐FeOOH) would transform into a pyrite‐type phase, a hydrogen‐bearing iron peroxide FeO 2 H x (with x from 0 to 1), with losing certain amount of hydrogen (Hu et al, ; Zhu et al, ) or without loss of any hydrogen (Nishi et al, ) at pressures >70–80 GPa upon laser heating.…”