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.