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Compressibility and phase stability of iron-rich ankerite

Abstract: 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. The rhombohedral ankerite structure is stable under compression up to 12 GPa. A third-order Birch?Murnaghan equation of state yields V0 = 328.2(3) Å3, bulk modulus B0 = 89(4) GPa, and its first-pressure derivative B?0 = 5.3(8)?values which are in good agreement with those obtained in our calculations for an ideal CaFe(CO3)2 ankerite composition. At 12 GPa, the iron-rich ankerite structure undergoes a reversible phase transition that could be a consequence of increasingly non-hydrostatic conditions above 10 GPa. The high-pressure phase could not be characterized. DFT calculations were used to explore the relative stability of several potential high-pressure phases (dolomite-II-, dolomite-III- and dolomite-V-type structures), and suggest that the dolomite-V phase is the thermodynamically stable phase above 5 GPa. A novel high-pressure polymorph more stable than the dolomite-III-type phase for ideal CaFe(CO3)2 ankerite was also proposed. This high-pressure phase consists of Fe and Ca atoms in sevenfold and ninefold coordination, respectively, while carbonate groups remain in a trigonal planar configuration. This phase could be a candidate structure for dense carbonates in other compositional systems.

 Autoría: Chuliá-Jordán R., Santamaria-Perez D., Ruiz-Fuertes J., Otero-De-la-roza A., Popescu C.,

 Fuente: Minerals 2021, 11(6), 607

 Editorial: MDPI

 Fecha de publicación: 06/06/2021

 Nº de páginas: 13

 Tipo de publicación: Artículo de Revista

 DOI: 10.3390/min11060607

 ISSN: 2075-163X

 Proyecto español: MAT2015-71070-REDC

 Url de la publicación: https://doi.org/10.3390/min11060607