Dolomite‐II: A high‐pressure polymorph of CaMg(CO₃)₂

We have measured the infrared spectra and x‐ray diffraction of CaMg(CO 3 ) 2 ‐dolomite to pressures of 50 GPa at 300 K. We observe both splittings and disappearances of x‐ray diffraction peaks initiating near 20 GPa, as well as new bands in the infrared spectrum of dolomite. The onset of the changes in both the x‐ray and infrared data appears to be gradual, and thus kinetically impeded: this is consistent with previous shock results. The infrared and x‐ray data are consistent with dolomite adopting a calcite‐III‐like structure. Our results indicate that high‐pressure polymorphism in dolomite could stabilize CaMg(CO 3 ) 2 in the deep mantle, and thus that high‐pressure polymorphs of dolomite could represent the main reservoir for carbon storage within Earth's lower mantle.