Compressibility and Phase Stability of Iron-Rich Ankerite

The structure of the naturally occurring, iron-rich mineral Ca1.08(6)Mg0.24(2)Fe0.64(4)Mn0.04(1)(CO3)2 ankerite was studied in a joint experimental and computational study. Synchrotron X-ray powder diffraction measurements up to 20 GPa were complemented by density functional theory calculations. rhombohedral is stable under compression 12 GPa. A third-order Birch–Murnaghan equation state yields V0 = 328.2(3) Å3, bulk modulus B0 89(4) GPa, its first-pressure derivative B’0 5.3(8)—values which are good agreement with those obtained our calculations for an ideal CaFe(CO3)2 composition. At undergoes reversible phase transition that could be consequence increasingly non-hydrostatic conditions above 10 high-pressure not characterized. DFT used explore relative stability several potential phases (dolomite-II-, dolomite-III- dolomite-V-type structures), suggest dolomite-V thermodynamically 5 novel polymorph more than dolomite-III-type also proposed. This consists Fe Ca atoms sevenfold ninefold coordination, respectively, while carbonate groups remain trigonal planar configuration. candidate dense carbonates other compositional systems.