Stability of Mullite as Derived from Equilibria in the System CoO‐Al 2 O 3 ‐SiO 2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO‐Al 2 O 3 ‐SiO 2 . Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO‐Al 2 O 3 permits calculation of ΔG° for the reaction 3Al 2 O 3 + 2SiO 2 = Al 6 Si 2 O 13 . The value obtained at 1422°C is ‐5.8 kcal.