New High-Pressure and High-Temperature CaCO3 Polymorph

In this work, we report an experimental observation of a new CaCO3 calcite-Vb phase, which occurs as the intermediate between aragonite and calcite-V at pressures >3.5 GPa and temperatures >1200 °C, suggesting it can exist in CaCO3-containing rocks at ∼125–200 km. Calcite-Vb is determined to have the KClO3-type structure with the monoclinic P21/m space group and lattice parameters of a = 6.284(5) Å, b = 4.870(3) Å, c = 3.991(4) Å, β = 107.94(3)°, and V = 116.19(15) Å3. Possible transformation mechanisms between calcite-Vb, aragonite, and calcite-V are illustrated.