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

Overview & Synopsis

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, \\\\\\\\rm\Ca\\\\\_\3\\\\left[\\\\\\rm\C\\\\\_\2\\\\\\\rm\O\\\\\_\5\\\right]\_\2\\\left[\\\\\\rm\CO\\\\\_\3\\\right]\\ 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 \\\\\\\\rm\Ca\\\\\_\3\\\\left[\\\\\\rm\C\\\\\_\2\\\\\\\rm\O\\\\\_\5\\\right]\_\2\\\left[\\\\\\rm\CO\\\\\_\3\\\right]\\ Ca 3 C 2 O 5 2 CO 3 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 \\\\\\\\rm\Ca\\\\\_\3\\\\left[\\\\\\rm\C\\\\\_\2\\\\\\\rm\O\\\\\_\5\\\right]\_\2\\\left[\\\\\\rm\CO\\\\\_\3\\\right]\\ Ca 3 C 2 O 5 2 CO 3 is characterized by the presence of \\\\\left[\\\\\\rm\CO\\\\\_\3\\\right]\^\2-\\\ CO 3 2 − - and \\\\\left[\\\\\\rm\C\\\\\_\2\\\\\\\rm\O\\\\\_\5\\\right]\^\2-\\\ C 2 O 5 2 − -groups. The results presented here imply that the formation of \\\\\\\\rm\Ca\\\\\_\3\\\\left[\\\\\\rm\C\\\\\_\2\\\\\\\rm\O\\\\\_\5\\\right]\_\2\\\left[\\\\\\rm\CO\\\\\_\3\\\right]\\ 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.