Compressibility and Phase Stability of Iron-Rich Ankerite

Pressure-driven phase transformations on Mg3Ca(CO3)4 huntite carbonate

Magnesium and calcium carbonate minerals are significant reservoirs of Earth's carbon and understanding their behavior under different conditions is crucial for elucidating the mechanisms of deep carbon storage. Huntite, Mg3Ca(CO3)4, is one of the two stable calcium magnesium carbonate phases, together with dolomite. The distinctive cation coordination environment of Ca atoms compared to calcite-type and dolomite structures makes huntite a comparatively less dense phase. Here we examine the behavior of a polycrystalline natural huntite sample under room-temperature compression up to 38 GPa. Synchrotron X-ray diffraction and Raman spectroscopy experiments were carried out in a diamond-anvil cell using He as a highly hydrostatic pressure transmitting medium. XRD results suggest that the initial R32 huntite structure persists up to 21 GPa. The Raman experiment agrees with this result but also suggests the appearance of structural defects from 10 GPa on. Birch-Murnaghan equation of state parameters were fit to the pressure-volume huntite data resulting in zero-pressure volume V0 of 611.7(2) Å3, a bulk modulus B0 of 99.5(11) GPa and a pressure derivative of the bulk modulus of . At 21 GPa, huntite transforms to another trigonal phase (R3), designated here as huntite II. This phase persists up to at least 38 GPa, the maximum pressure reached in this study. The major structural differences between huntite and the huntite-II phase involve the tilting of the [CO3] units with respect to the basal plane and a rotation, which cause a progressive change in the coordination number of the Ca atoms, from 6 to 9. DFT calculations complement the experimental data, providing new insights into the structural response to high-pressure conditions of this magnesium-calcium double carbonate mineral.

Structural Behavior of Minrecordite Carbonate Mineral upon Compression: Effect of Mg → Zn Chemical Substitution in Dolomite-Type Compounds

We report the structural behavior and compressibility of minrecordite, a naturally occurring Zn-rich dolomite mineral, determined using diamond-anvil cell synchrotron X-ray diffraction. Our data show that this rhombohedral CaZn0.52Mg0.48(CO3)2 carbonate exhibits a highly anisotropic behavior, the c axis being 3.3 times more compressible than the a axis. The axial compressibilities and the equation of state are governed by the compression of the [CaO6] and [ZnO6] octahedra, which are the cations in larger proportion in each layer. We observe the existence of a dense polymorph above 13.4(3) GPa using Ne as a pressure-transmitting medium, but the onset pressure of the phase transition decreases with the appearance of deviatoric stresses in nonhydrostatic conditions. Our results suggest that the phase transition observed in minrecordite is strain-induced and that the high-pressure polymorph is intimately related to the CaCO3-II-type structure. A comparison with other dolomite minerals indicates that the transition pressure decreases when the ratio Zn/Mg in the crystal lattice of pure dolomite is larger than 1. Density functional theory (DFT) calculations predict that a distorted CaCO3-II-type structure is energetically more stable than dolomite-type CaZn(CO3)2 above 10 GPa. However, according to our calculations, the most stable structure above this pressure is a dolomite-V-type phase, a polymorph not observed experimentally.

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate

The double carbonate BaCa(CO₃)₂ holds potential as host compound for carbon in the Earth’s crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO₃)₂ phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO₃)₂ polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO₃ and BaCO₃ end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.