Tetrahedrally coordinated carbonates in Earth's lower mantle

Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO$_3$. Using ab-initio calculations, we assign the new IR signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low pressure three-fold coordinated carbonates, as well as different chemical properties in the liquid state. Hence this may have significant implications on carbon reservoirs and fluxes and the global geodynamic carbon cycle.

Here, we report the first unequivocal evidence of tetrahedrally coordinated carbon in high pressure carbonates, obtained by a combined experimental and theoretical study. We perform in situ synchrotron infrared (IR) spectroscopic studies on ferromagnesite in diamond anvil cells (DAC) and identify a unique vibrational signature present only in the high pressure phase. We perform ab initio calculations of the IR spectra, which allow us to assign this vibrational signature to asymmetric, sp 3 -like C-O bonds.

In situ infrared spectroscopy characterization
Magnesite and siderite (FeCO 3 ) form a solid solution at ambient conditions and adopt the same structure at high pressure and temperature, except that upon substitution of Mg by Fe, the volume of the unit cell decreases by about 7% 6 (V(MgCO 3 ) = 351.7 Å 3 at 85 GPa and 2,400 ± 150 K and V(Mg 0.25 ;Fe 0.75 CO 3 ) = 328.9 Å 3 at 80 GPa and 300 K).We note that only the Febearing phase is temperature quenchable 6 , an important requirement for IR measurements in the DAC. We thus chose a natural sample of ferromagnesite with a composition (Mg 0. 25  into the post-magnesite phase when laser-heated at ~2100 K. In situ XRD was used in order to monitor the transformation into the post-magnesite phase ( Supplementary Fig. 1). IR spectra were recorded at the highest pressure of 103 GPa and during decompression of the postmagnesite phase back to 0 GPa at room temperature.
The mid-IR spectral absorption features result primarily from fundamental internal vibrations of the C-O bonds in the carbonate radical: the out-of-plane bending (ν 2 ), the asymmetric stretch (ν 3 ), and in-plane bending (ν 4 ) modes e.g. 11 . At ambient conditions, we measured these three modes on the polycrystalline carbonate phase (symmetry group: R-3C) at 867 cm -1 ; 1460 cm -1 and 739 cm -1 respectively ( Fig. 2A). Frequency of ν 3 was determined using a thinner sample (cf. supplementary Fig. 2). These frequencies are in good agreement with those reported by previous IR studies on iron-bearing carbonates e.g. 3,11,12 . Additional modes resulting from the combination of the three fundamental ones are also present: a band at 1811 cm -1 which stems from the combination of ν 1 +ν 4 ; and one at 2512 cm -1 which corresponds to the combination 2ν 2 +ν 3 13 .
Under compression at room temperature, no additional IR bands were observed; rather a shift to higher wave numbers was detected for all IR bands, except for the TO component of the ν 2 band which exhibited a slight negative pressure shift (-0.29 cm -1 /GPa). Such a shift is in agreement with previous studies on iron-bearing carbonates 3 , and has been interpreted as stemming from the increased strength of the divalent cation-oxygen bonds under compression. We observed no hysteresis upon decompression of the untransformed carbonate.
In the second set of experiments, several new IR bands were observed after transformation into the post-magnesite phase by laser heating ( Fig. 2A). At very high pressure (from 103 to 81 GPa) the most intense IR bands appeared to be saturated, which made their positions difficult to determine precisely, however our experiment showed that between 103 and 43 GPa these new IR modes gradually shift to lower wavenumbers (Fig. 2B) together with the post-magnesite phase. However, the new IR bands observed at ambient pressure after decompression cannot be assigned to magnetite e.g. 14,15 .

First principles calculations.
In order to interpret the IR spectrum measured for the post-magnesite phase, we carried out first principles calculations of the IR spectra of MgCO 3 at low and high pressure and we identified specific vibrational modes that are present only in the post-magnesite phase.
Calculation were conducted for pure MgCO 3 instead of the solid solution, (Mg,Fe)CO 3 , for computational simplicity. We used density functional theory (DFT), a semi local exchangecorrelation functional 16 , plane wave basis sets, and pseudopotentials. We first calculated the IR spectrum of magnesite (symmetry group: R-3C), the phase of MgCO 3 stable at ambient conditions (see Supplementary Fig. 3). We used a rhombohedral cell, with the lattice constant fixed at the experimental value of 5.675 Å and a cell angle of 48.2 17 (Supplementary Table 1).
The computed frequencies of the TO component of the ν 2 , ν 3 and ν 4 modes are 825, 1411, and 725 cm -1 respectively. The theoretical frequencies computed for a single crystal are lower than the experimental ones by ~ 4% 11,18 . This discrepancy is likely due to the use of the semi local exchange-correlation functional Perdew-Burke-Ernzerhof (PBE). By using the hybrid functional, B3LYP, Valenzano et al. 19 found a smaller error of ~0.5% for these three bands compared to the experimental results 11,18 . The IR spectrum of the same magnesite structure computed at a pressure of 83 GPa without allowing for any phase transition (Supplementary Table 1), shows that the TO component of the ν 2 mode is weakly modified (we found a modest blue shift of 5 cm -1 ), the ν 4 mode (TO) is blue-shifted to 913 cm -1 , and the ν 3 mode (TO) is blue-shifted to 1647 cm -1 ( Supplementary Fig. 4).  Table 3). The two bands measured at 890 and 987 cm -1 were well reproduced by our calculations; we found 908 and 987 cm -1 , respectively. Overall the theoretical and experimental spectra were in good agreement and, most importantly, they both exhibit a band at ~1300 cm -1 which is not present in magnesite at ambient conditions or in the IR spectrum of the isotropically compressed magnesite phase ( Supplementary Fig. 4). The compressed magnesite does not exhibit any extra band between 1000 and 1400 cm -1 , providing strong evidence that the experimental and theoretical band at ~1300 cm -1 signals the presence of a structural and bonding change.
Our geometry optimization of the tetrahedrally coordinated post-magnesite phase showed that the CO 4

Discussion
In summary, we report the first in situ characterization of carbon-oxygen bonds of the post-magnesite phase. We found that at high pressure, upon transformation into the postmagnesite phase, the IR spectrum of (Mg,Fe)CO 3 exhibits novel, unique features not present in the low pressure spectrum, which we assigned to a fundamentally different bonding configurations of carbon. Carbon bonds transforms from sp 2 (in the trigonal, doubly charged carbonate anions) to sp 3 hybridized configurations (in the tetrahedral tetra-charged carbonate anions), which we characterized using ab initio calculations within DFT. Hence our study provides the identification of a mode at 1304 cm -1 which may be used as a fingerprint of tetrahedrally bonded carbon in high pressure mineral phases. Carbon tetrahedrally bonded to oxygen has also been proposed to be present in high-pressure phase-V of CO 2 24,25 . However in this phase, tetrahedral CO 4 groups are symmetric with a C-O distance of 1.35Å, similar to the longest C-O distances observed here. Measured IR spectra showed the presence of a mode in the same frequency range at 1126 cm -1 , however all modes were reported to involve simultaneous stretching and bending, unlike the pure stretching mode 24 identified here.
The dramatic change we found in the carbon environment in ferromagnesite may have significant implications on carbon reservoirs and fluxes in the lower mantle and therefore on the deep carbon cycle. For example, the new bonding configuration, identified here in the highpressure carbonate structure is dramatically different from trigonal planar group in the ambient conditions structure. This will likely influence its chemical and physical properties such as its reactivity. Moreover, one would expect dramatic changes in the behavior of carbonate melts with increases in coordination of carbon owing to the ability of CO 4 to form polymerizable networks while CO 3 trigonal groups can not 26 . At upper mantle conditions, carbonate melts differ from silicate melts. They exhibit ultra low viscosity potentially resulting in high mobilities 27 .
Preliminary theoretical studies predict that carbonate melt viscosity increases at high pressures 28 which would inhibit mobility of carbonate melts in the lower mantle and might lead to the presence of deep carbon reservoirs.

Methods:
In situ high-pressure infrared spectroscopy: The infrared absorbance was measured in the 500 -4000 cm -1 range through a symmetric We carried out phonon calculations using density functional perturbation theory (DFPT) 33 to obtain the IR spectra. The intensity of the mth IR mode defined as 34 : where  and  are Cartesian components,