Calcium silicate-diorthocarbonate is a missing link in deep-mantle chemistry of silicates and carbonates.

Interactions between carbonates and silicates play a central role in many industrial and planetary processes. Although current knowledge is largely limited to silicate-carbonate exchange mechanisms, high-pressure compounds incorporating both carbonate and silicate units have long been anticipated. Here we report the synthesis of the high-pressure silicate-diorthocarbonate CaSiC2O7 at 122(2) GPa and 2800(200) K. CaSiC2O7 is based on the framework of vertex-sharing SiO6 octahedra and CO4 tetrahedra. Ab initio calculations are consistent with the experimental results and indicate dynamical and mechanical stability of this phase over a wide pressure range. These findings reveal an additional pathway for carbonate-silicate interaction in carbonate-rich deep-mantle environments. CaSiC2O7 has a density lower than that of major mantle minerals and exhibits comparatively low acoustic velocities, suggesting that silicate-carbonates may contribute to seismic anomalies near the core-mantle boundary.