Summary
The thermal breakdown of topaz, Al2SiO4(F, OH)2, has been studied, by X-ray diffraction, thermal analysis and electron microscopy. The results show the formation, at temperatures of 1000°C and above, of partly oriented mullite, (Al2O3)3–4 (SiO2)2, and silica. The silica formed during the reaction occurs as partly oriented cristobalite and/or as a siliceous glass, which at the temperature of the reaction must have been mobile, the minimum melting point in the silica-alumina system being reduced by the presence of fluorine. In certain cases, when hydroxyl-bearing topaz was heated to form mullite and subsequently reheated to higher temperatures, the mullite was replaced by partly oriented corundum.
The orientational control is believed to be by epitaxy on close-packed planes in the topaz. Mullite does not possess any planes which are described as close-packed, but consideration of the area of oxygen ion (treating each as a hard sphere) through which a plane cuts, gives a measure of the degree of approximation to close packing of planes within the structure, and helps to explain the mullite orientation. The orientation of the silica and corundum is consistent with the explanation of the orientation of the mullite.
A multi-stage reaction mechanism is proposed involving the dehydroxylation of hydroxylbearing topaz, the loss of fluorine by an inhomogeneous mechanism with the counterdiffusion of silicon and aluminium, and further loss of fluorine homogeneously.
Zusammenfassung
Die maximale thermische Stabilität von Topas Al2SiO4(F, OH)2 wurde mit Röntgendiffraktion, Differential-Thermoanalyse und Elektronenmikroskopie untersucht. Die Ergebnisse zeigen, daß bei Temperaturen von 1000°C und darüber teilweise orientierter Mullit (Al2O3)3–4 (SiO2)2 und SiO2-Phasen entstehen. SiO2 liegt einerseits als teilweise orientierter Cristobalit, andererseits auch in amorpher Form als Glas vor SiO2 muß bei den angegebenen Reaktionstemperaturen mobil gewesen sein; dies wird auf die Herabsetzung des Schmelzpunktes in Gegenwart von Fluor zurückgeführt. Bei erneuter Aufheizung in noch höhere Temperaturbereiche konnte in einigen Fällen ein Ersatz des Mullits durch teilweise orientierten Korund beobachtet werden.
Es wird angenommen, daß die Verwachsungen auf Epitaxie-Beziehungen an dichtestgepackten Netzebenen des Topas zurückgeführt werden können. Mullit besitzt keine dichtestgepackten Flächen; betrachtet man jedoch die Fläche der Sauerstoff-Ionen als “hard sphere”, die von einer Fläche geschnitten wird, so kann dies als Maß der Annäherung an dichteste Besetzung der Flächen innerhalb der Struktur gelten, und so zum Verständnis der Mullit-Orientierung beitragen. Die Orientierung von SiO2 und Korund stimmt mit dieser Interpretation überein.
Als Entstehungsmechanismus wird ein mehrstufiger Reaktionsablauf angenommen: Dehydroxylation des Topas, ein teilweiser Verlust von Fluor, verbunden mit einer gegenläufigen Diffusion von Silizium und Aluminium, und anschließend ein weiterer, homogener Verlust des Fluor.
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Hampar, M.S., Zussman, J. The thermal breakdown of Topaz. TMPM Tschermaks Petr. Mitt. 33, 235–252 (1984). https://doi.org/10.1007/BF01082671
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DOI: https://doi.org/10.1007/BF01082671