Novel Calcium sp³ Carbonate CaC₂O₅-I4̅2d May Be a Carbon Host in Earth’s Lower Mantle

Abstract

CaC2O5-I4̅2d was obtained by reacting CO2 and CaCO3 at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO4 4– groups form pyramidal [C4O10]4– complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC2O5-I4̅2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO2 under ambient conditions. As polymorphs of CaCO3 and CO2 are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC2O5-I4̅2d, which therefore needs to be considered instead of end-member CaCO3 in models of the mantle mineralogy.

High‐Pressure Crystal Structures of Pb2CO4 and PbC2O5 with Tetrahedral [CO4] and Pyrocarbonate [C2O5] atomic groups

“…The following earlier known structures have been used for the enthalpy calculations: PbCO 3 ‐ Pnma , [22] Pb 3 CO 5 ‐ Pnma , [23] Pb 2 CO 4 ‐ P 2 1 2 1 2 1 , [24] and PbCO 3 ‐ R 3 m [25] . In addition, we used structural models of Ca‐ and Sr‐carbonates with Ca and Sr replaced by Pb: Ca 3 CO 5 ‐ Cmcm , [1] Ca 3 CO 5 ‐ I 4/ mcm , [1] CaC 2 O 5 ‐ Pc , [7] CaC 2 O 5 ‐F dd 2, [1] CaC 2 O 5 ‐, [1] CaC 2 O 5 ‐ , [5] CaC 2 O 5 ‐ Cc , [7] and SrC 2 O 5 ‐ P 2 1 /c [6] . These structures were loaded as seeds for further crystal structure prediction using USPEX.…”

“…These compositions were chosen based on the analogy with other MOÀ CO 2 systems (M=Mg, Ca, Sr, Ba, and Mn), where crystal structures of only 3 : 1, 2 : 1, 1 : 1 and 1 : 2 are realized at high pressure both in theory [1][2][3][4] and experiment. [5,6,8,9,12,18] The search was performed for 1-4 formula units per unit cell. The size of the first generation in the USPEX calculations was equal to 65 structures.…”

“…Crystal structure prediction has been performed using evolutionary algorithms implemented in the USPEX 10.5 package [14–17] at 10, 25, and 50 GPa for compositions: Pb 3 CO 5 (3 : 1), Pb 2 CO 4 (2 : 1), PbCO 3 (1 : 1), PbC 2 O 5 (1 : 2), PbC 3 O 7 (1 : 3), and PbO. These compositions were chosen based on the analogy with other MO−CO 2 systems (M=Mg, Ca, Sr, Ba, and Mn), where crystal structures of only 3 : 1, 2 : 1, 1 : 1 and 1 : 2 are realized at high pressure both in theory [1–4] and experiment [5,6,8,9,12,18] …”

“…The sp 3 -hybridized I � 42d structure of this compound was experimentally synthesized at 34-45 GPa. [5] Recently, the crystal structure of pyrocarbonate SrC 2 O 5 with doubled (connected through the common oxygen atom) [CO 3 ] triangles was synthesized at 30 GPa. [6] After this finding, pyrocarbonate structure was also found for CaC 2 O 5 at pressure above 10 GPa.…”

“…Only mentioned CaC 2 O 5 compound was known until recent times. The sp 3 ‐hybridized structure of this compound was experimentally synthesized at 34–45 GPa [5] . Recently, the crystal structure of pyrocarbonate SrC 2 O 5 with doubled (connected through the common oxygen atom) [CO 3 ] triangles was synthesized at 30 GPa [6] .…”