Abstract Cristobalite and quartz react differently in mixtures with α-Al 2O 3 at 1415°C. With cristobalite, an eutectic liquid forms in accordance with the metastable phase equilibrium diagram for α-Al 2O 3-SiO 2 (cristobalite) in the absence of mullite. With quartz, a liquid first forms on the surfaces of quartz because of the occurence of an intermediate liquid phase on transformation of quartz to cristobalite. These liquids act as precursors to the formation of mullite by reacting with α-Al 2O 3. Mullite was detected earlier in the cristobalite-containing mixtures under similar firing conditions because the growth of mullite becomes significant with the formation of the eutectic liquid at the α-Al 2O 3-cristobalite interface since it is already saturated with Al 2O 3. The kinetics of sintering are affected by the rates of the step reactions.