Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO 2 ‐Al 2 O 3

Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO 2 ‐Al 2 O 3 solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO 2 ‐ and Al 2 O 3 ‐rich compositions. The calculated data were used to estimate regions of liquid‐liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of ∼1540 ° C at a critical composition of ∼36 mol% Al 2 O 3 was considered to be thermodynamically most probable; the gap extended from ∼11 to °49 mol% Al 2 O 3 at 1100 ° C. SiO 2 ‐rich glass compositions showed evidence of glass‐in‐glass phase separation when examined by direct transmission electron microscopy.