The properties of mullite-glass and mullite-mullite interfaces have been investigated at 1800 K by molecular dynamics (MD) simulation and high-resolution TEM. The simulation showed that mullite-glass interfaces typically have much lower interfacial energies than mullite-mullite interfaces, which results from the structural flexibility of the glass and associated accommodation of interfacial mismatch. The (110)-glass interface has the lowest energy of all interfaces studied, which is consistent with the observed dominance of this interface in experimental mullite-glass samples examined by TEM. The simulation shows that the interfacial energies of the (100)-glass and (010)-glass interfaces are higher than that those of the (001) Thus, the mullite structure is similar to that of sillimanite, which consists of AlO 6 octahedra chains that are parallel to the c axis and linked by SiO 4 and AlO 4 tetrahedra. During this reaction, the substitution of Si by Al is charge compensated by the removal of O and consequent O vacancy formation, which results in the disordering of the tetrahedrally-coordinated cations. 3,4 Sintered mullite, which commonly is formed by heating aluminosilicates (e.g