Na-Ca carbonates synthesized under upper-mantle conditions: Raman spectroscopic and X-ray diffraction studies

The new Na4Ca(CO3)(3), Na2Ca3(CO3)(4) and Na2Ca4(CO3)(5) compounds were synthesized in the system Na2CO3-CaCO3 in multianvil experiments at 6 GPa and characterized by Raman spectroscopy. In addition, the Na2Ca3(CO3)(4) compound was studied using in situ energy dispersive and single-crystal X-ray diffraction. Single bands in the CO32- symmetric stretching region (v(1)) and out-of-plane bending region (v(2)) in the Na4Ca(CO3)(3) Raman spectrum suggest a single crystallographically distinct carbonate group in the structure. In contrast, the spectra of Na2Ca3(CO3)(4) and Na2Ca4(CO3)(5) show two and three bands, respectively, in both the symmetric stretching region (v(1)) and out-of-plane bending region (v(2)), suggesting more than one crystallographically distinct carbonate group in the unit cell. Raman activity in the forbidden v(2) mode and multiple bands are observed in the in-plane bending region (v(4)) for the three compounds, proving the reduction of site symmetry of the CO32- ions with the loss of the threefold rotation axis (D-3h -> D-2h or C-s). Such a decrease in symmetry suggests distortion of the group itself, but may be attained by rearrangements of the coordinated metal cations as in the aragonite-group carbonates. At 6.5 GPa and 1000 degrees C, the structure of Na2Ca3(CO3)(4) was found to be orthorhombic or monoclinic with a beta angle close to 90 degrees and the lattice parameters: a = 7.3357(6) angstrom, b = 8.0377(9) angstrom, and c = 31.5322 (32) angstrom, with V = 929.59(14) angstrom(3). No structural changes were observed during pressure decrease down to 1 GPa, while a discontinuous increase in unit-cell parameters and volume was observed upon decompression from 1 GPa at room temperature. This indicates a pressure-induced phase transition to a structurally related ambient-pressure phase. The abnormally long c-parameter and proximity of the beta-angle to 90 degrees of Na2Ca3(CO3)(4) at ambient conditions suggest that, in the monoclinic system, the metric symmetry is higher than the Laue symmetry, which is a common sign for merohedral twinning.