Structure and elasticity of CaCO suggests carbonate contribution to the seismic anomalies of Earth's mantle.

Knowledge of carbonate compounds under high pressure inside Earth is key to understanding the internal structure of the Earth, the deep carbon cycle and major geological events. Here we use first-principles simulations to calculate the structure and elasticity of CaCO-minerals with different symmetries under high pressure. Our calculations show that CaCO-minerals represent a group of low-density low-seismic-wave velocity mantle minerals. Changes in seismic wave velocity caused by the phase transformation of CaCO-Cc to CaCO-I[Formula: see text]2d (CaCO-C2-l) agree with wave velocity discontinuity at a depth of 660 km in the mantle transition zone. Moreover, when CaCO-Fdd2 transforms into CaCO-C2 under 70 GPa, its shear wave velocity decreases by 7.4%, and its density increases by 5.8%, which is consistent with the characteristics of large low-shear-velocity provinces (LLSVPs). Furthermore, the shear wave velocity of CaCO-I[Formula: see text]2d is very similar to that of cubic Ca-perovskite, which is one of the main constituents of the previously detected LLSVPs. Therefore, we propose that CaCO and its high-pressure polymorphs may be a main component of LLSVPs.