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

Abstract

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.

Superlattice ordering transitions driven by short-range structure in barium calcium carbonates

“…Heating balcite did not produce Ba 0.5 Ca 0.5 CO 3 polymorphs alstonite, paralstonite, or barytocalcite. Alstonite and paralstonite have similar aqueous solubilities to barytocalcite 26 and similar calculated cohesive energies, 14 suggesting that they too should be more stable than balcite, which is unstable and nearly $20 kJ mol À1 above the energetic ground state. 12 The sequence of transformations alstonite / paralstonite / alstonite / balcomite suggests that alstonite is metastable relative to paralstonite at room temperature, but becomes more stable as the temperature increases.…”

“…11,12 However, simulations indicate that only a few of many possible structures occur naturally in the lithosphere despite relatively small energy differences between myriad metastable phases. 13 If differences in stability are small relative to the barriers to interconversion between phases, as they appear to be in the CaCO 3 -BaCO 3 system, 14 a better understanding of mineral transformation pathways may offer important insight into how these minerals are distributed and their implications for reconstructing past conditions. 15 Despite its persistence for months in aqueous solution, 12 balcite (Ba 0.5 Ca 0.5-CO 3 with R 3m symmetry) is unstable relative to calcite and witherite 11,12 and, putatively, to three naturally occurring Ba 0.5 Ca 0.5 CO 3 polymorphs (alstonite, paralstonite, barytocalcite).…”

High-Pressure Experimental and DFT Structural Studies of Aurichalcite Mineral

“…Nowadays, it is possible to recreate high-pressure (HP) and high-temperature (HT) conditions in the laboratory, and the sample can be characterized in situ by a battery of spectroscopic and diffraction techniques. Thus, numerous laboratory structural studies of simple and double carbonates at HP-HT have been reported in the last two decades [2][3][4][5][6][7][8][9][10][11][12][13][14]. The investigation of the HP-HT structural behavior of carbonate minerals bearing additional anion groups such as [PO 4 ] 3− in phosphate-carbonates [15], [SiO 4 ] 4− in silicate-carbonates [16][17][18] or [OH] − in basic carbonates [19][20][21] is, however, rather limited.…”

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

“…In this study, we conducted in situ high-pressure synchrotron XRD experiments of CaZn 0.52 Mg 0.48 (CO 3 ) 2 minrecordite up to 16 GPa to better understand the role played by the Mg 2+ to Zn 2+ cation substitution on the already complex polymorphism of double carbonates, − particularly on dolomite group carbonates, − at mantle pressures. DFT theoretical calculations on CaZn­(CO 3 ) 2 and CaZn 0.5 Mg 0.5 (CO 3 ) 2 stoichiometries provide information on the phase stability, compressibility, and anisotropy of minrecordite minerals.…”