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.
Highlights
The phase stability and elastic properties of minerals depend essentially on three intrinsic variables: the chemical composition (X), the pressure (P), and the temperature (T) [1,2]
Understanding the formation processes and stability of carbonate minerals under different thermodynamic conditions is crucial for Earth sciences and the geological carbon cycle
The indexation of our data confirmed a rhombohedral symmetry with lattice parameters a = 4.8361(9) Å and c = 16.185(3) Å (V = 327.83 (1) Å3), in excellent agreement with those expected from the Goldsmith linear regression formulae interrelating chemical composition and hexagonal lattice constants (a = 4.834 Å and c = 16.185 Å) [41], and those from previous studies on iron-rich ankerites [10,24]
Summary
The phase stability and elastic properties of minerals depend essentially on three intrinsic variables: the chemical composition (X), the pressure (P), and the temperature (T) [1,2]. Previous studies have demonstrated that carbonates of divalent metals with different radii and electronic characteristics present distinctive behaviors under compression and heating [6,7,8,9,10]. This is relevant within the complex scenario of Earth’s mantle, where natural compositions likely include the coexistence of several divalent cations
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