The crystal structure of dolomite-IV, a high-pressure polymorph of dolomite, at 115 GPa

We report the crystal structure of dolomite-IV, a high-pressure polymorph of Fe-dolomite stabilized at 115 GPa and 2500 K. It is orthorhombic, space group Pnma, a = 10.091(3), b = 8.090(7), c = 4.533(3) Å, V = 370.1(4) Å^3 at 115.2 GPa and ambient temperature. The structure is based on the presence of 3-fold C3O9 carbonate rings, with carbon in tetrahedral coordination. The structure of dolomite-IV has not been predicted, but it presents similarities with the structural models proposed for the high-pressure polymorphs of magnesite, MgCO3. A ring-carbonate structure match with spectroscopic analysis of high-pressure forms of magnesite-siderite reported in the literature, and, therefore, is a likely candidate structure for a carbonate at the bottom of the Earth’s mantle, at least for magnesitic and dolomitic compositions.