First-principles calculations of high-pressure iron-bearing monoclinic dolomite and single-cation carbonates with internally consistent Hubbard U

Abstract

It has been proposed that iron has a significant effect on the relative stability of carbonate phases at high pressures, possibly even stabilizing double-cation carbonates (e.g., dolomite) with respect to single-cation carbonates (e.g., magnesite, aragonite and siderite). X-ray diffraction experiments have shown that dolomite transforms at ~35 GPa to a high-pressure polymorph that is stable to decomposition; however, there has been disagreement on the structure of the high-pressure phase (Mao et al. in Geophys Res Lett 38, 2011. doi: 10.1029/2011GL049519; Merlini et al. in Proc Natl Acad Sci 109:13509–13514, 2012. doi: 10.1073/pnas.1201336109). Ab initio calculations interfaced with an evolutionary structure prediction algorithm demonstrated that a C2/c polymorph of pure CaMg(CO_3)_2 dolomite is more stable than previously reported structures (Solomatova and Asimow in Am Mineral 102:210–215, 2017, doi: 10.2138/am-2017-5830). In this study, we calculate the relative enthalpies up to 80 GPa for a set of carbonate phases including Fe-bearing solutions and endmembers, using the generalized gradient approximation and a Hubbard U parameter calculated through linear response theory to accurately characterize the electronic structure of Fe. When calculated with a constant U of 4 eV, the spin transition pressure of (Mg,Fe)CO_3 agrees well with experiments, whereas an internally consistent U overestimates the spin transition pressure by ~50 GPa. However, whether we use constant or internally consistent U values, a higher iron concentration increases the stability field of dolomite C2/c with respect to single-cation carbonate assemblages, but iron-free dolomite is not stable with respect to single-cation carbonates at any pressure. Thus, high-pressure polymorphs of Fe-bearing dolomite could in fact represent an important reservoir for carbon storage within oxidized sections of Earth's mantle.

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