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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 97, Issue 10

Issues

Second-order P6̄c2-P31c transition and structural crystallography of the cyclosilicate benitoite, BaTiSi3O9, at high pressure

Clivia Hejny
  • Corresponding author
  • Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria
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/ Ronald Miletich
  • Institut für Mineralogie und Kristallographie der Universität Wien, Althanstrasse 14, 1090 Wein, Austria
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/ Andreas Jasser
  • Institut für Geowissenschaften der Universität Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany
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/ Pascal Schouwink
  • Institut für Geowissenschaften der Universität Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany
  • Laboratoire de Cristallographie de l’Université de Genève, 24, quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
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/ Wilson Crichton / Volker Kahlenberg
  • Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria
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Published Online: 2015-04-02 | DOI: https://doi.org/10.2138/am.2012.4175

Abstract

Experimental high-pressure investigations on benitoite in the diamond-anvil cell reveal a secondorder phase transition at a critical transition pressure Pc = 4.24(3) GPa, as determined from synchrotron powder diffraction, single-crystal X-ray diffraction, and Raman spectroscopy. Diffraction experiments indicate a non-isomorphous transition from P6̄c2 to P31c space-group symmetry with a′ = a√3 and c′ = c relative to the P6̄2c subcell below Pc. The high-pressure polymorph is characterized by a larger compressibility compared to the compressional behavior of benitoite below Pc. Fitting second-order Birch-Murnaghan equations of state to the experimental data sets, the parameters obtained are V0 = 372.34(4) Å3, K0 = 117.9(7) GPa, with a0 = 6.6387(3) Å, Ka = 108.1(7) GPa, and c0 = 9.7554(4) Å, Kc = 143.3(1.1) GPa for the low-pressure form (P < Pc), and V0 = 376.1(4) Å3, K0 = 88.9(1.6) GPa, with a0 = 11.516(4) Å, Ka = 95.4(1.8) GPa, and c0 = 9.826(4) Å, Kc = 77.2(1.6) GPa for the high-pressure form (P > Pc). One of the most significant structural changes is related to the coordination of Ba atoms, changing from an irregular [6+6] coordination to a more regular ninefold. Simultaneously, the Si3O9 rings are distorted due to no longer being constrained by mirror-plane symmetry, and the Si atoms occupy three independent sites. The higher compressibility along the c-axis direction is explained by the relative displacement of the Ba position to the Si3O9 rings, which is coupled to the lateral displacement of the non-bridging O2-type atoms of the ring unit. A symmetry mode analysis revealed that the transition is induced by the onset of a primary order parameter transforming according to the K6 irreducible representation of P6̄c2.

Keywords : Benitoite; crystal structure; phase transition; high pressure; diamond-anvil cell; X-ray diffraction; Raman spectroscopy; symmetry mode analysis

About the article

Received: 2012-03-23

Accepted: 2012-06-19

Published Online: 2015-04-02

Published in Print: 2012-10-01


Citation Information: American Mineralogist, Volume 97, Issue 10, Pages 1749–1763, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2012.4175.

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