Carbonates, mostly presented in nature by calcite and aragonite (CaCO3), magnesite (MgCO3), dolomite (CaMg(CO3)2) and siderite (FeCO3), are widely distributed and play a crucial role in geological processes as weathering and diagenesis [1]. These carbonates have sp²-hybridized carbon atoms forming triangular [CO3] groups. Numerous experimental studies indicate that coordination of carbon atoms increases from triangular to tetrahedral (sp3) at high pressure, consistent with pressure-coordination rule [2].

Siderite, an anhydrous iron sp2-carbonate, decomposes at high temperatures and pressures above 75 GPa with the formation of sp3-carbonates [3]. However, the influence of water on the stability and chemistry of high-pressure iron carbonates remains poorly investigated, despite the widespread occurrence of hydrous carbonate phases in nature [4]. Recent studies indicate that hydrous carbonates (e.g., of barium) can be synthesized at elevated conditions [5].

Here, we report crystal structure determination of Fe5[CO4]3(OH), a hydrous iron sp3-carbonate, synthesized at 48(4) GPa and 2000-2200 K in laser-heated diamond anvil cells. The compound forms through reactions between different precursors belonging to Fe-C-O-H system: Fe2O3 - H2O2(aq) - C; FeCO3 - CO2 - H2O. Its crystal structure adopts an apatite-like framework, consistent with the DFT calculations. Our findings suggest that Fe5[CO4]3(OH) is stable under lower mantle conditions, indicating its potential role in deep Earth's water and carbon cycles.

References:

(1) Carbonates: Mineralogy and Chemistry; Reeder, R. J., Ed.; De Gruyter, 1983.

(2) Neuhaus, A.; Heide, H.; Steffen, R. In Festschrift für Leo Brandt zum 60. Geburtstag; Meixner, J., Kegel, G., Eds.; VS: Wiesbaden, 1968; pp 59–78.

(3) Cerantola, V.; et al. Nat Commun 2017, 8 (1), 15960.

(4) Hazen, R. M.; et al. In Carbon in Earth; Hazen, R. M., Jones, A. P., Baross, J. A., Eds.; De Gruyter, 2013; pp 7–46.

(5) Spahr, D.; et al. Chem. Commun. 2025, 61 (1), 161–164.