New Host for Carbon in the Planet

New research shows that a new Mg-Fe carbon-bearing compound can form at conditions corresponding to depths greater than 1,800 km.

Crystal structureThe global geochemical carbon cycle involves exchanges between Earth's interior and surface.  Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO2. 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. 

New research shows that a new Mg-Fe carbon-bearing compound can form at conditions corresponding to depths greater than 1,800 km [1]. Its structure, based on three-membered rings of corner-sharing (CO4)4- tetrahedra, closely agrees with predictions by first principles quantum calculations [2]. This  high-pressure polymorph of carbonates concentrates a large amount of Fe(III) as a result of intracrystalline reaction between Fe(II) and CO32- groups schematically written as 4FeO + CO2 -> 2Fe2O3 + C.  This results in an assemblage consisting of the new high-pressure phase, magnetite and diamond. 

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