Abstract: A refractory, porous ceramic composite including crystalline mullite (3Al2O3+2SiO2 or 3Al6Si2O11) and a crystalline phase of LaPO4 is formed from a mullite-LaPO4 sol-gel by annealing the dried gel. During the annealing process, particle sintering and self-foaming occur in the glassy state, and pores are produced due at least in part to the release of entrapped gases that form during the pyrolysis of the gel. The resulting crystalline composite, or crystalline nanocomposite, has a high porosity and is dimensionally and chemically stable at high temperatures. The composite also has a high degree of structural (e.g., mechanical) stability, related at least in part to the fine texturing and mixing of the mullite and LaPO4 during preparation of the sol. The resulting ceramic composite shows little or no shrinkage or expansion between about 1000C and about 1200° C.