The global geochemical carbon cycle involves exchanges between the Earth's interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO(2). The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing (CO(4))(4-) tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008) Novel high-pressure structures of MgCO(3), CaCO(3) and CO(2) and their role in Earth's lower mantle. Earth Planet Sci Lett 273:38-47]. This high-pressure polymorph of carbonates concentrates a large amount of Fe((III)) as a result of intracrystalline reaction between Fe((II)) and (CO(3))(2-) groups schematically written as 4FeO + CO(2) → 2Fe(2)O(3) + C. This results in an assemblage of the new high-pressure phase, magnetite and nanodiamonds.