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

Anton Shatskiy, Pavel N. Gavryushkin, Konstantin D. Litasov, Olga N. Koroleva, Igor N. Kupriyanov, Yuri M. Borzdov, Igor S. Sharygin, Kenichi Funakoshi, Yuri N. Palyanov and Eiji Ohtani
Na-Ca carbonates synthesized under upper-mantle conditions; Raman spectroscopic and X-ray diffraction studies
European Journal of Mineralogy (March 2015) 27 (2): 175-184

Abstract

The new Na (sub 4) Ca(CO (sub 3) ) (sub 3) , Na (sub 2) Ca (sub 3) (CO (sub 3) ) (sub 4) and Na (sub 2) Ca (sub 4) (CO (sub 3) ) (sub 5) compounds were synthesized in the system Na (sub 2) CO (sub 3) -CaCO (sub 3) in multianvil experiments at 6 GPa and characterized by Raman spectroscopy. In addition, the Na (sub 2) Ca (sub 3) (CO (sub 3) ) (sub 4) compound was studied using in situ energy dispersive and single-crystal X-ray diffraction. Single bands in the CO (sub 3) (super 2-) symmetric stretching region (v (sub 1) ) and out-of-plane bending region (v (sub 2) ) in the Na (sub 4) Ca(CO (sub 3) ) (sub 3) Raman spectrum suggest a single crystallographically distinct carbonate group in the structure. In contrast, the spectra of Na (sub 2) Ca (sub 3) (CO (sub 3) ) (sub 4) and Na (sub 2) Ca (sub 4) (CO (sub 3) ) (sub 5) show two and three bands, respectively, in both the symmetric stretching region (v (sub 1) ) and out-of-plane bending region (v (sub 2) ), suggesting more than one crystallographically distinct carbonate group in the unit cell. Raman activity in the forbidden v (sub 2) mode and multiple bands are observed in the in-plane bending region (v (sub 4) ) for the three compounds, proving the reduction of site symmetry of the CO (sub 3) (super 2-) ions with the loss of the threefold rotation axis (D (sub 3h) --> D (sub 2h) or C (sub 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 Na (sub 2) Ca (sub 3) (CO (sub 3) ) (sub 4) was found to be orthorhombic or monoclinic with a beta angle close to 90 degrees and the lattice parameters: a = 7.3357(6) Aa, b = 8.0377(9) Aa, and c = 31.5322 (32) Aa, with V = 929.59(14) Aa (super 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 Na (sub 2) Ca (sub 3) (CO (sub 3) ) (sub 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 27
Serial Issue: 2
Title: Na-Ca carbonates synthesized under upper-mantle conditions; Raman spectroscopic and X-ray diffraction studies
Affiliation: Russian Academy of Sciences, V. S. Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russian Federation
Pages: 175-184
Published: 201503
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 56
Accession Number: 2015-052249
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
Secondary Affiliation: Russian Academy of Sciences, Institute of Mineralogy, RUS, Russian FederationJapan Synchrotron Radiation Research Institute, JPN, Japan
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201524
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