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

Abstract

Investigations on the phase stability of the norsethite-type family (BaMg(CO 3 ) 2 , BaMn(CO 3 ) 2 , BaFe(CO 3 ) 2 ) under high-pressure conditions are of great significance for understanding the structure and metal cationic (Mg 2+ , Fe 2+ , Mn 2+ ) substitution mechanism in double divalent metal carbonates. The structural evolution and equation of state of BaFe(CO 3 ) 2 were studied firstly at high pressure up to ~7.3 GPa by synchrotron X-ray diffraction (XRD) in diamond anvil cell (DAC) in this study.BaFe(CO 3 ) 2 undergoes a reversible phase transition from R3 m (α-phase) to C2/c (γphase) space groups at ~3.0 GPa. The fitted elastic parameters are V 0 = 377.79(2) Å 3 and K 0 = 40.3(7) GPa for α-BaFe(CO 3 ) 2 , V 0 = 483.24(5) Å 3 and K 0 = 91.2(24) GPa for γ-BaFe(CO 3 ) 2 using second-order Birch-Murnaghan equation of state (BM2-EoS).Besides, the vibrational properties and structural stability of complete norsethite-type minerals were also investigated firstly by Raman spectroscopy combined with DAC up to 11.1 GPa. Similar structural phase transitions occur in BaMg(CO 3 ) 2 , BaFe(CO 3 ) 2 , BaMn(CO 3 ) 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)(CO 3 ) 2 and calcite-type (Mg,Fe,Mn)CO 3 carbonates. These results provide new insights into the divalent cation substitution effects on the stability and structural evolution of carbonates under high-pressure conditions.