New host for carbon in the deep Earth

(A) X-ray diffraction pattern collected at 80 GPa and room temperature of siderite transformed at 1,850–2,300 K. Crosses represent observed diffraction data after subtraction of the background and solid line the profile refinement. For the refinement we used an assemblage of high-pressure polymorph of magnetite (37) (Upper), untransformed siderite (space group R-3c) (Middle), and the new high-pressure phase (Lower). Residual between observations and fit is shown below the spectrum. (B) STEM high-angle annular dark field showing the untransformed siderite (Sid), the iron oxide (Mt for magnetite), and the transformed carbonate (HP carb.) appearing as a dark gray uniform matrix in the left side of the image. (C) EELS spectra collected on the recovered sample. These spectra provide qualitative information on the ratio of each phase (23, 24) and noticeable features have been indicated by small bars and can be compared with reference siderite, magnetite, and Fe^(III) oxide (22). Spectrum collected on the untransformed carbonate shows a high intensity peak of 707.7 eV that indicates the main iron speciation to be Fe^(II). In the case of the iron oxide, the broad L₃ peak at 707–709 eV with no splitting is characteristic of magnetite, whereas the L₂ shows many subsplitting and an intermediate energy position between pure ferric and ferrous iron that is typical of a mixed valence of magnetite (23). The spectrum collected in the new phase shows a L₃ line at higher energy loss than in carbonate groups with a fine structure indicating the main iron speciation to be Fe^(III). (D) C K-edge spectra collected in the untransformed and transformed carbonate phases. In the case of siderite relic, the peak at 290.3 eV corresponds to planar (CO₃)^2- carbonate groups. In the spectrum collected in the transformed carbonate, the slightly broader peak at 290.7 eV is attributed to the tetrahedral (CO₄)^4- forming rings of (C₃O₉)^6-. Presence of CO can also be detected in intimate association with the new phase.

(A) TEM image of the recovered sample from the ferropericlase + CO₂ experiment. Magnetite (Mt), high-pressure carbon-bearing phase (HP carb.), ferropericlase (FP), and nanodiamonds (D) are present. (B) C K-edge EELS spectrum of nanodiamonds observed in the recovered sample. The spectrum presents the absorption edge at 289 eV and the dip at 303 eV is characteristic of diamond C K-edge (38, 39).

Structure of the new high-pressure phase in space group P21/c related to phase II of magnesite proposed by theoretical calculation (15). (CO₄)^4- tetrahedra appear in green and magnesium atoms are shown as violet spheres.