Ca₃[C₂O₅]₂[CO₃] is a pyrocarbonate which can be formed at p, T-conditions prevalent in the Earth's transition zone

Understanding the fate of subducted carbonates is a prerequisite for the elucidation of the Earth's deep carbon cycle. Here we show that the concomitant presence of Ca[CO₃] with CO₂ in a subducting slab very likely results in the formation of an anhydrous mixed pyrocarbonate, $$ , at moderate pressure ( ≈ 20 GPa) and temperature ( ≈ 1500 K) conditions. We show that at these conditions $$ can be obtained by reacting Ca[CO₃] with CO₂ in a laser-heated diamond anvil cell. The crystal structure was obtained from synchrotron-based single crystal X-ray diffraction data. Density Functional Perturbation Theory calculations in combination with experimental Raman spectroscopy results unambiguously confirmed the structural model. The crystal structure of $$ is characterized by the presence of $$ - and $$ -groups. The results presented here imply that the formation of $$ needs to be taken into account when constructing models of the deep carbon cycle of the Earth.