Publication Date:
2015-03-27
Description:
The new Na 4 Ca(CO 3 ) 3 , Na 2 Ca 3 (CO 3 ) 4 and Na 2 Ca 4 (CO 3 ) 5 compounds were synthesized in the system Na 2 CO 3 –CaCO 3 in multianvil experiments at 6 GPa and characterized by Raman spectroscopy. In addition, the Na 2 Ca 3 (CO 3 ) 4 compound was studied using in situ energy dispersive and single-crystal X-ray diffraction. Single bands in the CO 3 2– symmetric stretching region ( v 1 ) and out-of-plane bending region ( v 2 ) in the Na 4 Ca(CO 3 ) 3 Raman spectrum suggest a single crystallographically distinct carbonate group in the structure. In contrast, the spectra of Na 2 Ca 3 (CO 3 ) 4 and Na 2 Ca 4 (CO 3 ) 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 CO 3 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 Na 2 Ca 3 (CO 3 ) 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) Å 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 β-angle to 90° of Na 2 Ca 3 (CO 3 ) 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.
Print ISSN:
0935-1221
Electronic ISSN:
1617-4011
Topics:
Geosciences
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