Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions.

Magnetite, Fe ₃ O ₄ , is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe ₃ O ₄ polymorphs, γ-Fe ₃ O ₄ with the orthorhombic Yb ₃ S ₄ -type structure and δ-Fe ₃ O ₄ with the modified Th ₃ P ₄ -type structure. The latter has never been predicted for iron compounds. The decomposition of Fe ₃ O ₄ at HPHT conditions was found to result in the formation of exotic phases, Fe ₅ O ₇ and Fe ₂₅ O ₃₂ , with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe ³⁺ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.