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

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[CO3] with CO2 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[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 $${{{{\rm{Ca}}}}}_{3}{\left[{{{{\rm{C}}}}}_{2}{{{{\rm{O}}}}}_{5}\right]}_{2}\left[{{{{\rm{CO}}}}}_{3}\right]$$ 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.