Diffusion couples, consisting of sapphire and fused silica, which were annealed in the temperature range from 1678° to 2003°C and analyzed by electron beam microprobe, have provided data on the stable phase equilibrium of the silica-alumina system. Under stable equilibrium conditions, the intermediate compound of this system, mullite (3Al2O3 · 2SiO2), melts incongruently at 1828° ± 10°C and its solid solution field extends from 70.5 to 74.0 percent (by weight) alumina. The stable phase diagram is a composite of the two binary eutectic diagrams: silica-mullite in the absence of alumina and silica-alumina in the absence of mullite. Under metastable conditions, mullite melts congruently at ≈1890°±10°C and its solid solution field extends to ≈83.2 percent (by weight) alumina.
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