Crystal chemistry and optics of bazzite from Furkabasistunnel (Switzerland) (original) (raw)

Summary

The crystal structure of blue bazzite (space group P6/mcc, Z = 2, α = 9.501(1),c = 9.178(1) Å) with the composition Be3(Sc1.25Fe3+ 0.43Mg0.32Mn0.03Al0.02)Σ=2.05 Si6O18[Na0.32(n H2O)] was refined from X-ray single-crystal data with 422 unique reflections to R = 2.2%. The structure is of the beryl type with octahedra strongly compressed parallel to the c-axis. The octahedral Me-O distance in bazzite is 2.080 A compared to 1.904 Å in beryl. The flattening of octahedra leads to a larger a cell dimension in bazzite compared to beryl (9.209 A). Two-valent cations (mainly Mg) in octahedral coordination are charge balanced by Na at (0, 0, 0) in the structural channels. Polarized single-crystal IR-spectra recorded between 400 and 8000 cm−1 indicate that H2O in the structural channels is oriented with the H-H vector perpendicular to the_c_-axis. The IR-spectra show more absorption bands than known for type II H2O in beryl thus the existence of more than one H2O species or even OH-groups is very likely.

Refractive indices of the same bazzite were measured using a spindle-stage and employing the wavelength-temperature variation method yielding no = 1.6279(3), ne = 1.6066(5) for 589 nm at 25°C. The birefringence Δ = 0.0213 is significantly larger than the one of near end-member beryl (Δ = 0.0047). This high birefringence of bazzite is related to the electronic polarizability of octahedral Sc and Fe which increase no at a stronger rate than ne. Transition metals in bazzite are also responsible for a higher refractive index dispersion than found for beryl.

Zusammenfassung

Die Kristallstruktur eines blauen Bazzits (Raumgruppe P6/mcc, Z = 2,a = 9.501(1),c = 9.178(1) Å) mit der Zusammensetzung Be3(Sc1.25Fe3+ 0.43Mg0.32Mn0.03Al0.02)Σ2=2.05 Si6)O18[Na0.32(n H2O)] wurde von Röntgeneinkristalldaten mit 422 Reflexen zu R = 2.2% verfeinert. Die Struktur ist die des Berylltyps mit der Besonderheit, daß die Oktaeder entlang der c-Achse stark zusammengedrückt sind. Die oktaedrischen Me-O Abstände im Bazzit betragen 2.080 Å, im Gegensatz zu 1.904Å im Beryll. Die Stauchung der Oktaeder im Bazzit führt zu einer längeren a-Achse als im Beryll (9.209 Å). Zweiwertige Kationen (vor allem Mg) in oktaedrischer Koordination werden durch Na auf (0, 0, 0), in den strukturellen Kanalen, neutralisiert. Polarisierte Einkristall-IR-Spektren zwischen 400 and 8000 cm−1 zeigen, daß H2O Moleküle in den strukturellen Kanalen mit dem H-H Vektor senkrecht zur c-Achse orientiert sind. Die IR-Spektren weisen außerdem mehr Absorptionsbanden auf, als für den H2O Typ II bekannt sind, sodaß entweder mehr als eine H2O Spezies oder sogar OH-Gruppen sehr wahrscheinlich sind.

Brechungsindizes am gleichen Bazzit wurden auf einem Spindeltisch mit der Wellenlängen-Temperatur Variationsmethode gemessen und ergaben no = 1.6279(3), ne = 1.6066(5) bei 589 nm und 25 °C. Die Doppelbrechung A = 0.0210 ist deutlich höher als für einen nahezu Endgliedberyll (A = 0.0047). Diese hohe Doppelbrechung des Bazzits wird durch die elektronische Polarisierbarkeit von oktaedrischem Sc und Fe verursacht, die no starker ansteigen läßt als ne. Übergangselemente im Bazzit sind auch für die höhere Dispersion der Brechungsindizes als im Beryll verantwortlich.

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Authors and Affiliations

1. Laboratorium für chemische und mineralogische Kristallographie, Universitat Bern, Freiestrasse 3, CH-3012, Bern, Switzerland
   Th. Armbruster
2. Institut für Mineralogie und Kristallographie, Universitat Wien, Dr.-Karl-Lueger-Ring 1, A-1010, Wien, Austria
   E. Libowitzky, M. Auernhammer, P. Bauerhansl, Ch. Hoffmann, E. Irran, A. Kurka & H. Rosenstingl
3. Mineralogisches Institut, Universitat Bern, Baltzerstrasse 1, CH-3012, Bern, Switzerland
   L. Diamond

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1. Th. Armbruster
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2. E. Libowitzky
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3. M. Auernhammer
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4. P. Bauerhansl
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5. Ch. Hoffmann
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6. E. Irran
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7. A. Kurka
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8. H. Rosenstingl
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Armbruster, T., Libowitzky, E., Auernhammer, M. et al. Crystal chemistry and optics of bazzite from Furkabasistunnel (Switzerland).Mineralogy and Petrology 52, 113–126 (1995). https://doi.org/10.1007/BF01163130

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• Received: 21 June 1993
• Accepted: 19 October 1993
• Issue Date: March 1995
• DOI: https://doi.org/10.1007/BF01163130

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