High‐Pressure Transformations and Stability of Ferromagnesite in the Earth's Mantle

Ferromagnesite (Mg,Fe)CO3 plays a key role in the transport and storage of carbon in the deep Earth. Experimental and theoretical studies demonstrated its high stability at high pressure and temperature against melting or decomposition. Several pressure-induced transformations of ferromagnesite have been reported at conditions corresponding to depths greater than ~1100 km in the Earth’s lower mantle. Although there is still no consensus on their exact crystallographic structures, evidences are strong of a change in carbon environment from the low-pressure planar CO32ion into carbon atoms tetrahedrally coordinated by four oxygens. High-pressure iron-bearing phases concentrate a large amount of Fe3+ as a result of intracrystalline self-redox reactions. These crystallographic particularities may have significant implications on carbon reservoirs and fluxes in the deep Earth.