Experimental Observation of a Calcium Silicon Double Carbonate

Reactions between carbon dioxide (CO₂) and silica (SiO₂), 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 [CO₃] groups and tetrahedrally coordinated silicate [SiO₄] units. In the CaO-SiO₂-CO₂ system, no phase had previously been shown to contain both octahedrally coordinated silicon [SiO₆] and carbonate [CO₃] groups, nor had such a structure been theoretically predicted. Here, we report the synthesis of the calcium silicon double carbonate Ca₂Si(CO₃)₄, obtained by reacting tilleyite, Ca₅(Si₂O₇)(CO₃)₂, with CO₂ 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 Ca₂Si(CO₃)₄ is unique among silicate-carbonates, featuring sixfold-coordinated Si sharing oxygen atoms with [CO₃] groups. Additionally, a novel calcium tetracarbonate, Ca₂(C₄O₁₀), containing tetrahedral [CO₄] 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.