A high-pressure structural transition of norsethite-type BaFe(CO₃)₂: Comparison with BaMg(CO₃)₂ and BaMn(CO₃)₂

Investigations on the phase stability of the norsethite-type family [BaMg(CO3)(2), BaMn(CO3)(2), BaFe(CO3)(2)] under high-pressure conditions are of great significance for understanding the structure and metal cationic (Mg2+, Fe2+, Mn2+) substitution mechanism in double divalent metal carbonates. The structural evolution and equation of state of BaFe(CO3)(2) were studied at high pressure up to similar to 7.3 GPa by synchrotron X-ray diffraction (XRD) in diamond-anvil cell (DAC) in this study. BaFe(CO3)(2) undergoes a reversible phase transition from R (3) over barm (alpha-phase) to C2/c (gamma-phase) space groups at similar to 3.0 GPa. The fitted elastic parameters are V-0 = 377.79(2) angstrom(3) and K-0 = 40.3(7) GPa for alpha-BaFe(CO3)(2), V-0 = 483.24(5) angstrom(3) and K-0 = 91.2(24) GPa for gamma-BaFe(CO3)(2) using second-order Birch-Murnaghan equation of state (BM(2)EoS). Besides, the vibrational properties and structural stability of complete norsethite-type minerals were also investigated first by Raman spectroscopy combined with DAC up to 11.1 GPa. Similar structural phase transitions occur in BaMg(CO3)(2), BaFe(CO3)(2), BaMn(CO3)(2) at 2.2-2.6, 2.6-3.7, and 3.7-4.1 GPa, respectively. The onset phase transition pressures of the norsethite-type family are much lower than that of dolomite-type Ca(Mg,Fe,Mn)(CO3)(2) and calcite-type (Mg,Fe,Mn)CO3 carbonates. These results provide new insights into the divalent cation substitution effects on the stability and structural evolution of carbonates under high-pressure conditions.