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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, Ca 3 C 2 O 5 2 CO 3 , at moderate pressure (≈ 20 GPa) and temperature (≈ 1500 K) conditions. We show that at these conditions Ca 3 C 2 O 5 2 CO 3 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 Ca 3 C 2 O 5 2 CO 3 is characterized by the presence of CO 3 2 − - and C 2 O 5 2 − -groups. The results presented here imply that the formation of Ca 3 C 2 O 5 2 CO 3 needs to be taken into account when constructing models of the deep carbon cycle of the Earth. Carbonates are transported into the deep Earth by subduction of the oceanic lithosphere, but the stability fields of subducted carbonates as a function of pressure, temperature, and composition remain incompletely described. Here, the authors synthesize the anhydrous, mixed pyrocarbonate Ca3[C2O5]2[CO3] from Ca[CO3] and CO2 in a laser-heated diamond anvil cell at moderate pressure and elucidate its structural features. [ABSTRACT FROM AUTHOR]