Na-Ca carbonates synthesized under upper-mantle conditions: Raman spectroscopic and X-ray diffraction studies
Shatskiy, Anton; Gavryushkin, Pavel N.; Litasov, Konstantin D.; Koroleva, Olga N.; Kupriyanov, Igor N.; Borzdov, Yuri M.; Sharygin, Igor S.; Funakoshi, Kenichi; Palyanov, Yuri N.; Ijiohtani, E
European Journal of Mineralogy Volume 27 Number 2 (2015), p. 175 - 184
published: Mar 1, 2015
ArtNo. ESP147052702007, Price: 29.00 €
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 CO3 2- symmetric stretching region (ν 1 ) and out-of-plane bending region (ν 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 (ν 1 ) and out-of-plane bending region (ν 2 ), suggesting more than one crystallographically distinct carbonate group in the unit cell. Raman activity in the forbiddenn ν 2 mode and multiple bands are observed in the in-plane bending region (ν 4 ) for the three compounds, proving the reduction of site symmetry of the CO3 2- 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°C, the structure of Na2Ca3(CO3)4 was found to be orthorhombic or monoclinic with a β angle close to 90° and the lattice parameters: a = 7.3357(6) Å, b = 8.0377(9) Å, and c = 31.5322 (32) Å, with V = 929.59(14) Å3. 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 β-angle to 90° 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.