title: The crystal chemistry of iron oxides and oxyhydroxides at extreme conditions : implications for the deep Earth's oxygen cycle
Creators: name: Koemets, Egor
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Fe-O
subject: FeOOH
subject: extreme conditions
subject: SC-XRD, DAC
subject: 550

description: This cumulative thesis describes an experimental investigation of the behavior of the iron-oxygen (Fe-O) system at conditions relevant to Earth’s lower mantle. The Fe-O system shows complex crystallochemical behavior at extreme conditions that impacts the properties of materials constituting Earth’s interior and redox processes operating within the planet. The high-pressure behavior of iron oxides plays a vital role in understanding the deep Earth since iron oxides represent the end‐members of materials that constitute Earth's mantle (FeO and Fe2O3 for ferrous (Fe2+) and ferric (Fe3+) states of iron in the mantle, respectively). Also, pure iron oxides and oxyhydroxides contained in banded iron formations can be transported into the lower mantle through the subduction of slabs; therefore, these phases are of great interest not only as model systems but also as natural samples involved in the dynamics of Earth. The thesis research aims to investigate the stability field of α-FeOOH (mineral goethite) at conditions of subducting slabs and describe the structural and chemical properties of high-pressure iron oxides, particularly FeO2, resulting from the decomposition of this material. The cornerstone technique used is in situ single-crystal X-ray diffraction (SC-XRD) in laser-heated diamond anvil cells (LH-DACs). Through structure solutions and refinements, this method gives the information on the crystal structure, composition, chemical bonding, and elastic properties of samples at extreme pressures and temperatures. Additional analytical techniques applied in determining the physical properties of various high-pressure Fe-O phases are Mössbauer and X-ray absorption spectroscopies. A series of experiments on FeOOH were performed at 40-107 GPa and 1200-2500 K to cover the likely conditions inside subducting slabs being transported into the lower mantle. Firstly, the methodological aspects of selecting a proper pressure-transmitting medium for experiments with FeOOH were addressed. The popular NaCl pressure-transmitting medium reacted with FeOOH and therefore contaminated the Fe-O system. As a result of chemical reaction between NaCl and FeOOH at 107(2) GPa and 2000(200) K, a novel orthorhombic Na2FeCl4OHx phase was formed (space group #55, Pbam). The formation of such a compound, even in small quantities, is detrimental: its presence affects the chemistry of Fe-O system and complicates the interpretation of spectroscopic data. Therefore, in subsequent experiments on FeOOH, a neon (Ne) pressure-transmitting medium was used. The stability field of α-FeOOH loaded in Ne was investigated in a series of experiments at 40-82 GPa and 1200-2500 K. FeOOH decomposed at these conditions, forming a variety of pure high-pressure iron oxides with release of water and oxygen. Observed among the decomposition products were the already known high-pressure iron oxide phases ι-Fe2O3, η-Fe2O3 HP-Fe3O4, Fe5O7, and FeO2Hx, as well as several new phases. We studied these novel phases, solved and refined their structures: Fe7O10 (space group Cmcm) and Fe6.32O9 (space group P63/m). We also made a detailed study on their structural changes at various pressures via cold compression and decompression (compressibility and iron spin state transitions). The decomposition of FeOOH indicates the instability of this phase in subducting slabs at conditions corresponding to 1000 km depth. Our results suggest that the recycling of FeOOH in Earth’s mantle leads to a release of oxidizing fluids, causing a change in redox conditions. We hypothesize that the transport of these fluids (or oxidized mantle material) to Earth’s surface could explain the sporadic increase of oxygen levels before the Great Oxidation Event, which is linked to the formation of large igneous provinces. Our hypothesis implies that the formation of Earth’s oxygen-rich atmosphere resulted from processes in the early anaerobic biosphere, plate tectonics, and decomposition of ferric iron oxyhydroxides in Earth’s mantle. A detailed study of FeO2Hx was performed, which was recently reported in the literature as a “pyrite-type” iron peroxide. We also synthesized FeO2 isostructural to FeO2Hx from direct interaction between iron and oxygen to explore the properties of these materials with a focus on defining the oxidation state of iron and Fe-O bonding features. It was found that FeO2 and FeO2Hx do not belong to the pyrite-type structure, but belong instead to HP-PdF2 type. Using a multimethodological approach that included Mössbauer spectroscopy, XANES spectroscopy, and advanced theoretical analysis (DFT + DMFT), we discovered surprising results: cubic FeO2 and FeO2Hx contain ferric (3+) iron and oxygen has the formal oxidation state 1.5-. This observation required a reconsideration of the crystal chemistry of the major Earth’s mantle phases, (Fe,Al)-bearing silicate perovskite (bridgmanite) and post-perovskite, that revealed considerable changes in their behavior at conditions of the lowermost mantle. We observed the deviation of the oxidation state of oxygen (2-) in these phases. These effects may change our understanding of geochemical and geophysical processes in both the lower mantle and on a global scale.