Ca[CO][CO] 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[CO] with CO 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[CO] with CO 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.