AbstractThe presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO₃ partially dissociates to form various iron oxides. At higher pressures FeCO₃ forms two new structures—tetrairon(III) orthocarbonate Fe₄³⁺C₃O₁₂, and diiron(II) diiron(III) tetracarbonate Fe₂²⁺Fe₂³⁺C₄O₁₃, both phases containing CO₄ tetrahedra. Fe₄C₄O₁₃ is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.