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 P2₁2₁2) 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 V₀ = 258.4(3) Å³, K₀ = 62(1) GPa; and V₀ = 304.8(3) Å³, K₀ = 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) × V₀], with V₀ in cm³/mol, and the aragonite-structure to post-aragonite-structure phase transition pressures increase with decreasing ionic radius of the cations.