Abstract
Dolomite is one of the major mineral forms in which carbon is subducted into the Earth’s mantle. End-member CaMg(CO3)2 dolomite typically breaks down upon compression into two carbonates at 5–6 GPa in the temperature range of 800–1200 K (Shirasaka et al. 2002). However, high-pressure X-ray diffraction experiments have shown that dense high-pressure polymorphs of dolomite may be favored over single-cation carbonates (Santillán et al. 2003; Mao et al. 2011; Merlini et al. 2012). Here we compare calculated dolomite structures to experimentally observed phases. Using density functional theory interfaced with a genetic algorithm that predicts crystal structures (USPEX), a monoclinic phase with space group C2/c was found to have lower energy at pressures above 15 GPa than all previously reported dolomite structures. It is possible that this phase is not observed experimentally due to a large activation energy of transition from dolomite I, resulting in the observed second-order phase transition to a metastable dolomite II. Due to the complex energy landscape for candidate high-pressure dolomite structures, several structurally unique metastable polymorphs exist. We calculate the equation of state of a set of lowest-energy dolomite polymorphs with space groups
Acknowledgements
We thank E.A. Schauble, A. Kavner, M. Merlini, G.F. Finkelstein, A.R. Oganov, and O. Hellman for valuable discussions and insights. We are thankful to N. Near-Ansari for assistance with technical aspects using FRAM, the high-performance computing cluster at Caltech. This work is supported by the U.S. National Science Foundation through award EAR-1551433.
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