SYNTHESIS OF A NEW HIGH-PRESSURE AND HIGH-TEMPERATURE IRON OXIDE, FE4O5, A PLAUSIBLE KEY PLAYER IN DEEP EARTH PROCESSES

Iron oxides have broad geophysical and geochemical relevance due to their abundance, electronic properties and role in redox equilibria. While investigating the stability and decomposition products of siderite (FeCO₃) at conditions of the Earth’s mantle we obtained, at 10 GPa and about 1800 K, a single crystal of unknown phase. Diffraction data were obtained using highly focused synchrotron x-rays. Structural solution and refinement of the single crystal diffraction pattern indicate that the phase is a new compound with formula Fe₄O₅ ^[1]. The phase has orthorhombic symmetry where iron is 6-coordinated in layers of trigonal prisms and edge-sharing octahedra. First-principle calculations show that Fe₄O₅ is stable at high pressure with respect to its possible breakdown products. We performed several HP-HT syntheses starting from mixtures of pure Fe+Fe₃O₄ and Fe+Fe₂O₃. Fe₄O₅ was synthesized in the range 10-20 GPa readily upon heating at temperature in the range 1500-2500 K. Upon decompression and further heating we observed the decomposition of Fe₄O₅ at ~ 5GPa and 1800K, to wüstite and magnetite; whereas cold decompression shows that orthorhombic Fe₄O₅ is a phase retrievable to ambient conditions.

The new Fe-O compound has a broad fundamental and applicative significance, spanning planetary science, physics, chemistry and materials science. Also, the new iron oxide is a plausible phase of deep Earth, considering that it is rather reduced and taking into account the high heterogeneity of the upper mantle. Many peridotite from deep (>200-300km) subduction zones contain olivine and garnet rich in Fe₃O₄ lamellae exsolution which attest to the decompression of precursor phases that had formed at profound depths preceding mantle upwelling^[2]. Microdiamonds from UHP gneisses also contain abundant Fe_xO_y nanometric inclusions whose structures and stoichiometries are not known yet, though the depth of the host rock subduction is determined as >250 km^[3]. These examples and recent findings of iron oxides coexisting with wüstite and ferrites as inclusions in superdeep diamonds from Brazil^[4] suggest potential areas for searching for natural Fe₄O₅ within terrestrial rocks.

^[1] Lavina et al. PNAS 2011. ^[2] Zhang et al. Am Mineral 1999. ^[3] Dobrzhinetskaya, Gondwana Res, 2012. ^[4] Wirth et al. EOS Transactions AGU 2009.