Experimental Observation of a Calcium Silicon Double Carbonate

Abstract Reactions between carbon dioxide (CO 2 ) and silica (SiO 2 ), as well as between carbonates and silicates, are central to understanding carbon behavior in planetary interiors and have important technological implications. Yet, only a few oxides are known in which carbon and silicon coexist within the same crystal lattice. These silicate–carbonates typically contain trigonal carbonate [CO 3 ] groups and tetrahedrally coordinated silicate [SiO 4 ] units. In the CaO–SiO 2 –CO 2 system, no phase had previously been shown to contain both octahedrally coordinated silicon [SiO 6 ] and carbonate [CO 3 ] groups, nor had such a structure been theoretically predicted. Here, we report the synthesis of the calcium silicon double carbonate Ca 2 Si(CO 3 ) 4 , obtained by reacting tilleyite, Ca 5 (Si 2 O 7 )(CO 3 ) 2 , with CO 2 in diamond anvil cells at 39.5 GPa and 2600 K. The compound was subsequently temperature‐ and pressure‐quenched, remaining metastable at ambient conditions. Structural and spectroscopic characterization was performed using synchrotron single‐crystal X‐ray diffraction and Raman spectroscopy. The structure of Ca 2 Si(CO 3 ) 4 is unique among silicate–carbonates, featuring sixfold‐coordinated Si sharing oxygen atoms with [CO 3 ] groups. Additionally, a novel calcium tetracarbonate, Ca 2 (C 4 O 10 ), containing tetrahedral [CO 4 ] units, was discovered. These findings reveal a new oxide chemistry under extreme conditions and open avenues for synthesizing metastable carbon‐bearing materials relevant to the deep carbon cycle.