Chemical Stability of FeOOH at High Pressure and Temperature, and Oxygen Recycling in Early Earth History

Goethite, $α$-FeOOH, is a major phase among oxidized iron species, commonly called rust. We studied the behavior of iron (III) oxyhydroxide up to 81 GPa and 2100 K using in situ synchrotron single-crystal X-ray diffraction. At high pressure-temperature conditions FeOOH decomposes forming oxygen-rich fluid and different mixed valence iron oxides (previously known phases of Fe$_2$O$_3$, Fe$_3$O$_4$, Fe$_5$O$_7$, and novel Fe$_7$O$_{10}$ and Fe$_{6.32}$O$_9$). Rust is known to form as a byproduct of anoxygenic prokaryote metabolism that took place massively from about 3.8 billion years (Ga) ago until the Great Oxidation Event (GOE) ∼2.2 Ga ago. Rust was buried on the ocean floor and was transported into the mantle as a consequence of plate tectonics (started ∼2.8 Ga ago). Our results suggest that recycling of rust in Earth's mantle contributes to redox conditions of the early Earth and formation of oxygen-rich atmosphere