Summary
The crystal structure of elbaite has been refined on a crystal from San Diego County, California (USNM R-17011), Na1.69Mn0.45Ca0.42□0.38B0.05 K0.01)(Al4.78Li3.74Mn0.39Fe 2+0.09 ) Al18.00B9.00(Si17.94B0.06) [O82.57(OH)8.62F1.81] to a weightedR of 4.2% for 1142 observed, symmetry-independent reflections. It differs very little from the other refined tourmaline structures, mainly in the somewhat smaller Y and Z octahedra and in the nearly hexagonal Si6O18 ring of tetrahedra. The mechanism of solid solution in tourmalines stems from the necessary adjustment, in size and shape, between edge-sharing Y and Z octahedra that are chemically and crystallographically distinct. Mössbauer studies show that iron occupies both Y and Z positions in ferric as well as ferrous oxidation states. This makes it possible for the mica-like building blocks, composed of Y octahedra and Si6O18 rings, to be joined together by Z octahedra. In the absence of Fe, with mainly Al, Li and Mg as substituting cations, the critical adjustment between Y and Z octahedra becomes impossible, and the tourmaline structure cannot form along the elbaite-dravite join.
Zusammenfassung
Die Kristallstruktur des Elbaits ist mit Hilfe eines Einkristalles von San Diego County, Kalifornien (USNM R-17011), Na1, 69Mn0,45Ca0,42 □0,38B0,05 K0,01) (Al4,78Li3,74Mn0,39Fe 2+0,09 ) Al18,00B9,00(Si17,94B0,06) [O82,57(OH)8,62 F1,81] bestimmt worden. Für 1142 beobachtet symmetrieunabhängige Reflexe ist das gewichtete Residuum 4,2%. Die Elbait-Struktur unterscheidet sich nur wenig von den andern genau bestimmten Turmalin-Strukturen; der Unterschied liegt in etwas kleineren Y- und Z-Oktaedern und einem fast genau hexagonalen Ring von Si6O18-Tetraedern. Der Mangel an fester Lösung zwischen Dravit und Elbait wird durch die kritische Größe und Gestalt der chemisch und kristallographisch verschiedenen Y-und Z-Oktaedern erklärt, die sich in einer gemeinsamen Kante treffen. Das Mössbauer-Studium von Turmalin-Strukturen zeigt, daß sowohl Z-wie Y-Punktlagen von Fe2+- sowie Fe3+- Atomen besetzt sind. Diese Kationen von verschiedener Größe ermöglichen das Zusammenfügen von Y- und Z-Gruppen. Wenn nur Al-, Li- und Mg-Kationen für die Mittelpunkte von Y- und Z-Oktaeder zur Verfügung stehen, kann die Turmalinstruktur nicht entstehen.
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With a Chemical Analysis byC. O. Ingamells, U.S.G.S., Menlo Park, California, U.S.A.
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Donnay, G., Barton, R. Refinement of the crystal structure of elbaite and the mechanism of tourmaline solid solution. TMPM Tschermaks Petr. Mitt. 18, 273–286 (1972). https://doi.org/10.1007/BF01082837
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DOI: https://doi.org/10.1007/BF01082837