Unraveling the complexity of iron oxides at high pressure and temperature: Synthesis of Fe(5)O(6)

The iron-oxygen system is the most important reference of rocks’ redox state. Even as minor components, iron oxides can play a critical role in redox equilibria, which affect the speciation of the fluid phases chemical differentiation, melting, and physical properties. Until our recent finding of Fe(4)O(5), iron oxides were assumed to comprise only the polymorphs of FeO, Fe(3)O(4), and Fe(2)O(3). Combining synthesis at high pressure and temperature with microdiffraction mapping, we have identified yet another distinct iron oxide, Fe(5)O(6). The new compound, which has an orthorhombic structure, was obtained in the pressure range from 10 to 20 GPa upon laser heating mixtures of iron and hematite at ~2000 K, and is recoverable to ambient conditions. The high-pressure orthorhombic iron oxides Fe(5)O(6), Fe(4)O(5), and h-Fe(3)O(4) display similar iron coordination geometries and structural arrangements, and indeed exhibit coherent systematic behavior of crystallographic parameters and compressibility. Fe(5)O(6), along with FeO and Fe(4)O(5), is a candidate key minor phase of planetary interiors; as such, it is of major petrological and geochemical importance. We are revealing an unforeseen complexity in the Fe-O system with four different compounds—FeO, Fe(5)O(6), Fe(4)O(5), and h-Fe(3)O(4)—in a narrow compositional range (0.75 < Fe/O < 1.0). New, finely spaced oxygen buffers at conditions of the Earth’s mantle can be defined.