Novel high-pressure calcium carbonates

Abstract

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO 2 and CaO-CO 2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO 3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO 3 , and newly predicted Ca 3 CO 5 and CaC 2 O 5 . Ca 3 CO 5 polymorphs are found to contain isolated orthocarbonate CO 4 4tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC 2 O 5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO 4 -tetrahedra. Detailed analysis of chemical stability of CaCO 3 , Ca 3 CO 5 and CaC 2 O 5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO 3 is the main host of oxidized carbon throughout the lower mantle.

Postaragonite phases of CaCO3 at lower mantle pressures

“…1 are in very good agreement with other DFT results. [22,23,26] Beyond the stability field of P nma aragonite, we observe two competing monoclinic structures − the P 2 1 /c-l structure from Ref. [25].…”

“…However, further structure searches using AIRSS predicts a CaCO 3 structure in the megabar regime with a difference − a P 2 1 /c unit cell with pyroxene chains stacked out-ofphase, in contrast to the parallel chains in the C222 1 structure. [25] This difference reduces enthalpy significantly [25,26] and is accompanied by a marked difference in Raman signature, which was used recently by Lobanov et al to confirm P 2 1 /c-h (here, the label -h originates from Ref. [25] and refers to the higher pressure of the two predicted P 2 1 /c phases with sp 3 hybridization) as the stable structure for deep mantle CaCO 3 alongside X-ray diffraction.…”

Carbon sequestration during core formation implied by complex carbon polymerization

“…The shift in carbon coordination from predominantly three-fold to four-fold is reminiscent of carbon behavior in crystalline carbonates. At ambient conditions, natural carbonates contain three-fold carbon; however, in high-pressure polymorphs of CaCO 3 and MgCO 3 , carbon increases its coordination to four-fold at 75 and 83 GPa, respectively 22,23 while in Ca 3 CO 5 , a newly-proposed theoretical carbonate polymorph, fourfold carbon appears at pressures as low as 11 GPa 24 . Thus, the pressure range of the transition from CO 3 to CO 4 in silicate melts encompasses the predicted or observed coordination transition pressures in crystalline carbonate phases.

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Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle

“…3d, e), indicating the sample was sufficiently heated to transform starting materials to high-pressure silicate structures, but no carbonate-silicate exchange reaction occurs. Two hypotheses can explain these observations: (1) in contrast to theoretical predictions that the reversal of the carbonate-exchange reaction takes place at higher pressures and lower temperatures [26][27][28][29][30] , CaCO 3 + MgSiO 3 become more favorable than MgCO 3 + CaSiO 3 from 91 to 137 GPa and 2100 to 2800 K; (2) the reaction CaC-to-MgC is hindered by reaction kinetics, and metastable starting materials are observed. Fig.…”

“…Previous experiments 24,25 indicate that CaCO 3 reacts with silicates to form MgCO 3 via the forward reaction up to 80 GPa and 2300 K, i.e., at least to the mid-lower mantle. Theoretical studies further predict that MgCO 3 + CaSiO 3 are enthalpically favored over CaCO 3 + MgSiO 3 throughout the lower mantle pressure and temperature regime [26][27][28][29][30] . However, although many studies have addressed the stability of individual carbonates up to higher pressures [31][32][33] , no experiments examined the carbonate-silicate cation exchange reaction up to core-mantle boundary conditions.…”