The crystal chemistry of iron oxides and oxyhydroxides at extreme conditions : implications for the deep Earth's oxygen cycle

Abstract

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 Mossbauer 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 group55, 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…