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High‐Temperature, High‐Pressure Reactions of H2 with CaCO3 Melts

First‐principles molecular dynamics calculations are performed to investigate the reactions of hydrogen and calcium carbonate melts at pressure–temperature conditions appropriate to the Earth's lower mantle and core–mantle boundary. Two models with different hydrogen‐to‐carbonate ratios are studied. A variety of chemical reactions are observed. It is found that hydrogen dissociates readily and reacts with free carbonate anions forming various transient chemical species and water molecules. Further reactions of these reactive species serve as intermediates to form C–C and C–O connections. The unreacted bulk carbonates are linked via polymeric‐cornered shared CO4 tetrahedra. At 110 GPa and 4087 K, “diamondoids” with tetrahedral C4 moieties are found. The theoretical results support recent reports on the observation of tetrahedra CO4 in high‐pressure carbonate glasses and suggest a plausible explanation of ice VII inclusion in deep‐Earth diamonds.