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American Mineralogist; May 2008; v. 93; no. 5-6; p. 918-927; DOI: 10.2138/am.2008.2744
© 2008 Mineralogical Society of America
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Crystal structure of synthetic Al4B2O9: A member of the mullite family closely related to boralsilite

Reinhard X. Fischer1,*, Volker Kahlenberg2, Dietmar Voll3, Kenneth J.D. MacKenzie4, Mark E. Smith5, Bernhard Schnetger6, Hans-Jürgen Brumsack6 and Hartmut Schneider1

1 Fachbereich Geowissenschaften, Universität Bremen, Klagenfurter Strasse, D-28359 Bremen, Germany
2 Institut für Mineralogie and Petrographie, Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria
3 Institut für Mineralogie und Kristallographie, Universität Wien—Geozentrum, Althanstrasse 14, A-1090 Wien, Austria
4 School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
5 Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K.
6 Institut für Chemie und Biologie des Meeres (ICBM), Universität Oldenburg, Carl von Ossietzky Strasse, D-26111 Oldenburg, Germany

Correspondence: * E-mail: rfischer{at}uni-bremen.de

The crystal structure of Al4B2O9, synthesized from Al(NO)3·9H2O and B(OH)3 via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with 11B and 27Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al32B16O72 is closely related to the boralsilite (Al32B12Si4O74) structure with Si replaced by B and to mullite (Al4+2xSi2–2xO10–x). It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) Å, b = 5.5413(2) Å, c = 15.0531(6) Å, β = 90.913(2)°, Z = 8 for Al4B2O9. The main structural units are isolated chains of edge-sharing AlO6-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO4, AlO5, BO3, and BO4 groups with two B atoms and one O atom (O5') disordered on interstitial positions. 27Al and 11B NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold:[fourfold + fivefold] = ~50%:50%) and of threefold and fourfold coordinated B (~80%:20%).

Key Words: Aluminum borate • boron aluminate • Al4B2O9 • boralsilite • crystal structure • Rietveld refinement • MAS NMR spectroscopy






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