New High‐Pressure Phase of CaCO₃ at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

In this study, we have investigated the stability of CaCO 3 at high pressures and temperatures using synchrotron X‐ray diffraction in laser‐heated diamond anvil cells. Our experimental results have shown that CaCO 3 in the aragonite structure transforms into CaCO 3 ‐VII ( P 2 1 / c ) at 27 GPa and 1,500 K with a negative Clapeyron slope of −4.3(9) MPa/K. CaCO 3 ‐VII is stable between 23 and 38 GPa at 2,300 K and transforms into post‐aragonite at 42 GPa and 1,400 K. Furthermore, it reacts with stishovite, an abundant form of SiO 2 in subducted oceanic crust, forming CaSiO 3 ‐perovskite. The occurrence of CaSiO 3 ‐perovskite via the reaction of CaCO 3 ‐VII and stishovite provides an explanation for the observation of the high concentrations of CaSiO 3 ‐perovskite and some amount of CaCO 3 in deep‐mantle inclusions. CaCO 3 ‐VII is thus an important carbon‐bearing phase at the topmost lower mantle and may provide necessary carbon to produce deep‐mantle diamonds.