Complexity of High-Pressure Orthorhombic Iron Oxides, the Characterization of Fe5O6

Abstract:

Occurring as accessory minerals in most rocks and forming large deposits of considerable economical importance, iron oxides have a major petrological importance. Their role as oxygen buffers, in differentiation processes and as magnetic phases summarize the critical importance of iron oxides in most petrological contexts, independently of their abundance.
The discovery of a new compound in the Fe-O system, Fe₄O₅[1], reshaped our assumptions on the behavior of iron oxides in the Earth’s deep interior, where phases of FeO and Fe₃O₄ were considered the sole plausible players. Further studies found that Fe₄O₅ is stable in a wide compositional range[2] and can accept a wide extent of isomorphic substitutions[3].
We used laser heating synthesis in diamond anvil cell and microdiffraction mapping with high brilliance synchrotron x-ray[4] to explore the complexity of the Fe-O system at high pressure and temperature. We found coexistence of two to three oxides in most of the samples we investigated. By means of a careful exploration of diffraction effects in the reciprocal space, we singled-out the diffraction peaks of a few grains in multiphase diffraction patterns. These allowed a reliable characterization of yet a new iron oxide, Fe₅O₆. This compound, synthesized between 10 and 20 GPa, is also orthorhombic and can be described with the same building blocks of the other known orthorhombic iron oxides. A comparison of compressibility and lattice parameters of the latest iron oxides will be presented.

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[2] Woodland, A. B., Frost, D. J., Trots, D. M., Klimm, K. & Mezouar, M. In situ observation of the breakdown of magnetite (Fe₃O₄) to Fe₄O₅ and hematite at high pressures and temperatures. Am Mineral 97, 1808–1811 (2012).
[3] Woodland, A. B. et al. Fe₄O₅ and its solid solutions in several simple systems. Cotrib Mineral Petrol 166, 1677–1686 (2013).
[4] Lavina, B., Dera, P. & Meng, Y. Synthesis and microdiffraction at extreme pressures and temperatures. e50613 (2013).