The reconstructive 2/1- to 3/2-mullite transformation in the presence of Fe2O3-rich glass at 1570 °C
Schneider, Hartmut; Pleger, Roland
European Journal of Mineralogy Volume 5 Number 3 (1993), p. 515 - 522
published: Jun 14, 1993
manuscript accepted: Dec 28, 1992
manuscript received: Nov 29, 1991
ArtNo. ESP147050503017, Price: 29.00 €
Abstract Reaction couples consisting of mullite single-crystal discs cut perpendicular either to  or to  and of Fe2O3-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 heat-treatment of sample specimens produces small volumes of 3/2-mullite from the original 2/1-mullite near the glass-mullite interface. The penetration depth of 3/2-mullite depends on the orientation of the 2/1 single crystals relative to the coexisting melt and varies from ≈15 μm parallel  and ≈25 μm parallel . Electron diffraction and dark-field patterns indicate that the 2/1- to 3/2-mullite transformation takes place with preservation of the mullite bulk structure, in spite of the fact that major structural rearrangement and atomic diffusion are required. Though the formation of 3 /2-mullite out of 2/1-mullite reflects reaction to a more stable state at the given temperature, the extremely low 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 2/1- 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 % Fe2O3. With increasing distance from the glass-mullite interface, the Fe2O3 content decreases below 0.1 mole % within ≈ 50 μm in profiles parallel to , and within ≈80 μm parallel to , respectively. Although the substitution of Fe for Al and the 2/1- 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 Fe2O3-rich glass promotes atom exchange processes, thus helping to surmount the high activation energy of the structural transformation.