Pressure-induced phase transition of CO32−-bearing scapolite by in situ X-ray diffraction and vibrational spectroscopy

Scapolite Na_2.25Ca_1.43K_0.30Fe_0.02[Al_4.23Si_7.77O₂₄]Cl_0.72(SO₄)_0.1(CO₃)_0.18 has been investigated by single-crystal X-ray diffraction, Infrared and Raman spectroscopy combined with diamond anvil cells (DAC) up to 19.6 GPa at room temperature, to understand its phase stability and the behaviors of CO₃²⁻. The experimental results show that a phase transition from the tetragonal phase (P4₂/n) to the triclinic phase occurs at 10.7 GPa, which is attributed to the pressure-induced rotation of tetrahedra around the bridging O atoms. The pressure–volume data were fitted to the second-order Birch–Murnaghan equation of state, yielding V₀ = 1102.6(8) ų and K₀ = 70.3(9) GPa for the tetragonal phase and V₀ = 1188.2(16) ų and K₀ = 33.0(3) GPa for the triclinic phase. In the triclinic phase, the larger compressibility is due to the increased degree of freedom in the crystal structure, and the anisotropy of the axial compressibility may be related to the ionic radius of the anionic group. From the Infrared absorption and Raman spectroscopy data, we speculate that CO₃²⁻ is extruded by the four-membered rings at the (001) plane during the whole pressure range. The high-pressure behavior of CO₃²⁻ in scapolite provides a possibility that carbon exists in the Earth’s interior as CO₃²⁻ coupled to silicates.