High-pressure polymorphism and structural transitions of norsethite, BaMg(CO₃)₂

Author(s)

T. Pippinger, Ronald Miletich-Pawliczek, H. Effenberger, Gregor Hofer, P. Lotti, M. Merlini

Abstract

In situ high-pressure investigations on norsethite, BaMg(CO₃)₂, have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P_c ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C2/c symmetry (Z = 4; a = 8.6522(14) Å, b = 4.9774(13) Å, c = 11.1542(9) Å, β = 104.928(8)°, V = 464.20(12) ų at 3.00 GPa), and the structure refinement (R₁ = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO₃ groups were found to get tilted off the ab-plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K₀ = 66.2 ± 2.3 GPa and dK/dP = 2.0 ± 1.8 for α-norsethite and K₀ = 41.9 ± 0.4 GPa and dK/dP = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β _a^-1 = 444(53) GPa, β _c^-1 = 76(2) GPa; γ: β _a^-1 = 5.1(1.3) × 10³ GPa, β _b^-1 = 193(6) GPa β _c^-1 = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius-Clapeyron slope of 9.8(7) × 10^-3 GPa K^-1. An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated (Formula presented.) space-group symmetry. The same superstructure reflections, which disappear as temperature increases, were found to gain in intensity due to the positional shift of the Ba atoms in the γ-phase.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Università degli studi di Napoli Federico II, Università degli Studi di Milano-Bicocca

Journal

Physics and Chemistry of Minerals

Volume

41

Pages

737-755

No. of pages

19

ISSN

0342-1791

DOI

https://doi.org/10.1007/s00269-014-0687-8

Publication date

05-2014

Peer reviewed

Yes

Austrian Fields of Science 2012

105113 Crystallography, 104011 Materials chemistry, 105116 Mineralogy

Keywords

ASJC Scopus subject areas

General Materials Science, Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/55eed2b8-e97a-43f3-ba2c-8b12a41bf010