Ferromagnesite (Mg,Fe)CO 3 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 ~1030 km in the Earth's lower mantle. Although there is still no consensus on their exact crystallographic structures, evidence is strong for a change in carbon environment from the low-pressure planar CO 3 2-ion into carbon atoms tetrahedrally coordinated by four oxygens. High-pressure iron-bearing phases concentrate a large amount of Fe 3+ as a result of intra-crystalline self-redox reactions. These crystallographic par-ticularities may have significant implications on carbon reservoirs and fluxes in the deep Earth.