Structures and stability of calcium and magnesium carbonates at mantle pressures

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate $({\mathrm{CaCO}}_3$ and ${\mathrm{MgCO}}_3)$ at high pressures. We find a previously unknown ${\mathrm{CaCO}}_3$ structure which is more stable than the aragonite and “post aragonite” phases in the range 32–48 GPa. At pressures from 76 GPa to well over 100 GPa the most stable phase is a previously unknown ${\mathrm{CaCO}}_3$ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of ${\mathrm{MgCO}}_3$ in the range 85–101 GPa. Our results lead to a revision of the phase diagram of ${\mathrm{CaCO}}_3$ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of ${\mathrm{MgCO}}_3$. We predict ${\mathrm{CaCO}}_3$ to be more stable than ${\mathrm{MgCO}}_3$ in the Earth's mantle above 100 GPa, and that ${\mathrm{CO}}_2$ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.

Figure 1

Enthalpies per f.u. of ${\mathrm{CaCO}}_3$ phases relative to “post aragonite,” with the number of f.u. per primitive unit cell given within square brackets. The enthalpies of phases known prior to the current study are shown as dashed lines, while those found in the current study are shown as solid lines. The dotted red line shows the collapse of the ${\mathrm{CaCO}}_3\text{−}P{2}_1/c\text{−}\mathrm{l}$ structure into the more stable ${\mathrm{CaCO}}_3\text{−}P{2}_1/c\text{−}\mathrm{h}$ structure at 80–90 GPa.

Figure 2

Enthalpies per f.u. of ${\mathrm{MgCO}}_3$ phases relative to the $C2/m$ phase, with the number of f.u. per primitive unit cell given within square brackets. Previously known phases are shown as dashed lines, and those found in the current study are shown as solid lines.

Figure 3

The ${\mathrm{CaCO}}_3\text{−}P{2}_1/c\text{−}\mathrm{l}$ (top), ${\mathrm{CaCO}}_3\text{−}Pnma\text{−}\mathrm{h}$ (middle), and “post aragonite” (bottom) structures of ${\mathrm{CaCO}}_3$ at 40 GPa. The Ca atoms are in green, the carbon in gray, and the oxygen in red.

Figure 4

The $C{222}_1$ (top) and ${\mathrm{CaCO}}_3\text{−}P{2}_1/c\text{−}\mathrm{h}$ pyroxene-type (bottom) structures of ${\mathrm{CaCO}}_3$ at 60 GPa. The Ca atoms are in green, carbon in gray, and oxygen in red.

Figure 5

X-ray diffraction patterns of the $C{222}_1$[15] and ${\mathrm{CaCO}}_3\text{−}P{2}_1/c\text{−}\mathrm{h}$ phases of ${\mathrm{CaCO}}_3$, compared with experimental data from Fig. 1(b) of Ref. [34]. Data at 182 GPa are reported, with an incident wavelength of 0.415 Å. The stars indicate that the peak immediately to the right arises from platinum.

Figure 6

The relative stabilities per f.u. as a function of pressure of ${\mathrm{MgCO}}_3+{\mathrm{SiO}}_2$ and ${\mathrm{MgSiO}}_3+{\mathrm{CO}}_2$. The vertical gray line indicates the pressure at the base of the mantle (136 GPa). In this and the following figures, the kinks arise from phase transitions.

Figure 7

The relative stabilities per f.u. as a function of pressure of ${\mathrm{CaSiO}}_3+{\mathrm{CO}}_2$ and ${\mathrm{CaCO}}_3+{\mathrm{SiO}}_2$. The vertical gray line indicates the pressure at the base of the mantle (136 GPa).

Figure 8

Enthalpy per f.u. of ${\mathrm{CaCO}}_3+{\mathrm{MgSiO}}_3$ compared with that of ${\mathrm{CaSiO}}_3+{\mathrm{MgCO}}_3$. Below 100 GPa we find that ${\mathrm{CaSiO}}_3+{\mathrm{MgCO}}_3$ is the most stable, while above 100 GPa ${\mathrm{CaCO}}_3+{\mathrm{MgSiO}}_3$ is the most stable.