Volatiles, such as carbon and water, modulate the Earth's mantle rheology, partial melting and redox state, thereby playing a crucial role in the Earth's internal dynamics. We experimentally show the transformation of goethite FeOOH in the presence of CO 2 into a tetrahedral carbonate phase, Fe 4 C 3 O 12 , at conditions above 107 GPa-2300 K. At temperatures below 2300 K, no interactions are evidenced between goethite and CO 2 , and instead a pyrite-structured FeO 2 H x is formed as recently reported by Hu et al. (2016; 2017) and Nishi et al. (2017). The interpretation is that, above a critical temperature, FeO 2 H x reacts with CO 2 and H 2 , yielding Fe 4 C 3 O 12 and H 2 O. Our findings provide strong support for the stability of carbon-oxygen-bearing phases at lower-mantle conditions. In both subducting slabs and lower-mantle lithologies, the tetrahedral carbonate Fe 4 C 3 O 12 would replace the pyrite-structured FeO 2 H x through carbonation of these phases. This reaction provides a new mechanism for hydrogen release as H 2 O within the deep lower mantle. Our study shows that the deep carbon and hydrogen cycles may be more complex than previously thought, as they strongly depend on the control exerted by local mineralogical and chemical environments on the CO 2 and H 2 thermodynamic activities.