9 JUNE 2016 | VOL 534 | NATURE | 241 LETTER doi:10.1038/nature18018 FeO 2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles Qingyang Hu 1,2 *, Duck Young Kim 1,2 *, Wenge Yang 1,3 *, Liuxiang Yang 1,3 , Yue Meng 4 , Li Zhang 1,2 & Ho-Kwang Mao 1,2 The distribution, accumulation and circulation of oxygen and hydrogen in Earth’s interior dictate the geochemical evolution of the hydrosphere, atmosphere and biosphere 1 . The oxygen- rich atmosphere and iron-rich core represent two end-members of the oxygen–iron (O–Fe) system, overlapping with the entire pressure–temperature–composition range of the planet. The extreme pressure and temperature conditions of the deep interior alter the oxidation states 1 , spin states 2 and phase stabilities 3,4 of iron oxides, creating new stoichiometries, such as Fe 4 O 5 (ref. 5) and Fe 5 O 6 (ref. 6). Such interactions between O and Fe dictate Earth’s formation, the separation of the core and mantle, and the evolution of the atmosphere. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget, with hydrogen having a key role in the reaction of Fe and O (causing iron to rust in humid air). Here we use first-principles calculations and experiments to identify a highly stable, pyrite-structured iron oxide (FeO 2 ) at 76 gigapascals and 1,800 kelvin that holds an excessive amount of oxygen. We show that the mineral goethite, FeOOH, which exists ubiquitously as ‘rust’ and is concentrated in bog iron ore, decomposes under the deep lower-mantle conditions to form FeO 2 and release H 2 . The reaction could cause accumulation of the heavy FeO 2 -bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles. This process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic O 2 source for the Great Oxidation Event over two billion years ago that created the present oxygen-rich atmosphere. We started with α-Fe 2 O 3 (haematite) powder loaded in cryogeni- cally condensed liquid O 2 in the sample chamber of a diamond-anvil cell (DAC) (see Methods). The pressure was initially raised to 78 GPa; no reaction between haematite and O 2 was observed at ambient tem- perature (∼293 K). Using a Nd-doped Y 3 Al 5 O 12 laser system 7 to heat the sample to 1,800 K in situ at high pressure, the sample became semi-transparent (Fig. 1b), suggesting that a chemical reaction had occurred. The X-ray diffraction (XRD) pattern shows new sets of sharp, single crystal-like diffraction spots (Fig. 1a) that are readily distinguishable from the original broad and smooth texture of the Fe 2 O 3 powder pattern. Integration of the diffraction spots (Fig. 2a) shows eight peaks that do not match any known Fe 2 O 3 (refs 3, 4) or O 2 phases 8 , but can be unambiguously indexed to a rather simple cubic structure (Fig. 1c) with the space group Pa3 (Table 1). The spotty XRD pattern is ideally suited to the multigrain crystal- lography method 9 recently adopted for high-pressure research 10,11 . The spots are treated as diffraction from multiple single crystals, and sorted according to individual crystal orientation matrices. At least 33 single crystallites were identified by the multigrain crystallography method software. All symmetry-allowed spots for Pa3 are present, and all observed spots can be accounted for by the Pa3 unit cell. The details of five crystallites are presented in Extended Data Tables 1 and 2. The new phase has a structure identical to that of pyrite (FeS 2 ) with oxygen replacing sulphur, the next-row chalcogen element. Results from Rietveld refinement are shown in Fig. 2. For this structure, oxygen atoms not only form O–Fe bonds of 1.792 Å, but also O–O bonds of 1.937 Å (Extended Data Fig. 1 and Extended Data Table 3), that are typical of peroxide. Analogous to the archetypical pyrite, the iron in FeO 2 is considered to be ferrous. Curiously, the oxidation of Fe 2 O 3 to FeO 2 reduces Fe 3+ to Fe 2+ . This can be understood with the concurrent oxidation of O 2- to O 2- and O 0 as indicated by the O–O bond. In other words, this material can be viewed as FeO holding extra O 2 . We shall refer to the pyrite phase of Pa3 peroxide as the P-phase. To assess the stability of the P-phase under pressure, we calculated the volume change of the reaction at 76 GPa as follows: + = () 2Fe O O 4FeO 1 2 3 2 2 Here we used molar volumes of 35.69 Å 3 for Fe 2 O 3 in the Aba2 structure 4 , 12.79 Å 3 for O 2 in the O 8 cluster 12 , and 20.76 Å 3 for the P-phase. The reaction has a volume shrink of ΔV/V = -1.4%. Pressure lowers the Gibbs free energy of the reaction by ΔG = ∫ΔVdP, thus favouring the formation of FeO 2 at increasing pressure. The P-phase is non-quenchable to ambient conditions; its XRD peaks dis- appear below 31 GPa during pressure release at 300 K (Extended Data Fig. 2). We expanded our study from the O–Fe binary to the O–Fe–H ter- nary, and showed that the P-phase could also be synthesized under moderately reducing conditions coexisting with H 2 . We studied the Fe 2 O 3 –H 2 O join, in which the most stable compound FeOOH occurs ubiquitously as rust on Earth’s surface, in the deep ocean, on meteor- ites, on other planets, and on moons, in the α-, β-, γ-, δ-, or ε-FeOOH forms. It concentrates in bog iron ore deposits, which have been used as a copious, renewable resource of iron ever since the Iron Age. The α-FeOOH (goethite) transforms to the ε-phase at high pressure and decomposes to Fe 2 O 3 + H 2 O at high temperature. Its pressure–temper- ature phase boundaries and pressure–volume–temperature equations of state have been previously determined up to 29.4 GPa and 523 K (ref. 13). We compressed goethite in Ne pressure medium to 92 GPa and laser-heated it to 2,050 K. XRD clearly shows the conversion to the P-phase (Fig. 2b), indicating the following reaction: = + () 2FeOOH 2FeO H 2 2 2 The H 2 at 92 GPa and 2,050 K is far above its melting temperature of 900 K (ref. 14), and the H 2 fluid is highly mobile. Raman spec- troscopy is used to search for H 2 , and clearly observed H 2 vibron peaks at 5,180 cm -1 (Fig. 3), corresponding to H 2 in the Ne pressure medium 15 . The production of free H 2 indicates a moderately reducing condition. 1 Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, China. 2 Geophysical Laboratory, Carnegie Institution, Washington DC 20015, USA. 3 High Pressure Synergetic Consortium (HPSynC), Geophysical Laboratory, Carnegie Institution, Argonne, Illinois 60439, USA. 4 High Pressure Collaborative Access Team (HPCAT), Geophysical Laboratory, Carnegie Institution, Argonne, Illinois 60439, USA. * These authors contributed equally to this work. © 2016 Macmillan Publishers Limited. All rights reserved