Abstract
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(3)] with CO(2) in a subducting slab very likely results in the formation of an anhydrous mixed pyrocarbonate, Ca3(C2O5)2(CO3) , at moderate pressure ( ≈ 20 GPa) and temperature ( ≈ 1500 K) conditions. We show that at these conditions Ca3(C2O5)2(CO3) can be obtained by reacting Ca[CO(3)] with CO(2) 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 Ca3(C2O5)2(CO3) is characterized by the presence of (CO3)2- - and (C2O5)2- -groups. The results presented here imply that the formation of Ca3(C2O5)2(CO3) needs to be taken into account when constructing models of the deep carbon cycle of the Earth.