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Reaction couples consisting of mullite single-crystal discs cut perpendicular either to [010] or to [001] and of Fe203-rich glass layers were annealed at 1570 °C for 4d. Reaction phenomena in mullite were studied by electron microprobe analysis and by transmission electron microscopy. The he at-treatment of sampie specimens produces small volumes of 3/2-mullite from the original 2/ I-mullite near the glass-mullite interface. The penetration depth of 3/2-mullite depends on the orientation of the 211 single crystals relative to the coexisting melt and varies from '" 15 11m parallel [010] and ",25 11m parallel [001]. Electron diffraction and dark-field patterns indicate that the 211-to 3 /2-mullite transformation takes place with preservation of the mullite bulk structure, in spite of the fact that major structural rear rangement and atomic diffusion are required. Though the formation of3/2-mullite out of 211-mullite reflects reaction to a more stable state at the given temperature, the extremely 10w mobility of diffusion species prevents a solid-state 2/1-to 3/2-mullite transformation in measurable times. A (partial) transformation can be achieved only in the presence of a coexisting liquid phase. The 211-to 3/2-mullite transformation is accompanied by a Fe for Al substitution in mullite. Near the glass-mullite interface, mullite incorporates up to '" 1.2 mole % Fe203. With increasing distance from the glass-mullite interface, the Fe203 content decreases below 0.1 mole % within '" 50 11m in profiles parallel to [010], and within ",80 11m parallel to [001], respectively. Although the substitution of Fe for Al and the 211-to 3/2-transformation take place simultaneously in mullite, no structural interdependence between the two processes is believed to exist. However, the presence of the low-viscosity Fe203-rich glass promotes atom exchange processes, thus helping to surmount the high activation energy of the structural transformation.

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