High-Pressure Phase Transitions And Compressibilities Of Aragonite-Structure Carbonates: Srco3 And Baco3

The aragonite-structure carbonates-strontianite (SrCO3) and witherite (BaCO3)-were investigated by synchrotron X-ray diffraction combined with diamond anvil cells up to 30 and 15 GPa at room temperature, respectively. Phase transitions in SrCO3 (Pmcn to P2(1)2(1)2) and BaCO3 (Pmcn to Pmmn) were observed at 22.2-26.9 and 9.8-11.2 GPa, respectively. Both strontianite and witherite display anisotropic linear compression under pressure, with the c-axis 2-3 times more compressible than the a-axis and b-axis. The obtained second-order Birch-Murnaghan equation of state parameters for strontianite and witherite are V (0) = 258.4(3) a"<<(3), K (0) = 62(1) GPa; and V (0) = 304.8(3) a"<<(3), K (0) = 48(1) GPa, respectively. Based on the current results for strontianite and witherite and previous data for aragonite (CaCO3) and cerussite (PbCO3), the bulk moduli of the aragonite-structure carbonates exhibit a linear correlation with ambient molar volume [K (T0) (GPa) = 138 (5) - 2.0 (3) x V (0)], with V (0) in cm(3)/mol, and the aragonite-structure to post-aragonite-structure phase transition pressures increase with decreasing ionic radius of the cations.