The mullite chemical formula is 3AlO3·2SiO2, and the theoretical composition is Al2O3 71.8%, SiO2 28.2%, which is the only stable compound in the AI2O3-SiO2 binary system and the most widely studied crystal phase. Its melting point is (1890 ± 90) ° C, crystallized in an orthorhombic crystal form. With the increase of Al3+ content, more oxygen vacancies will be generated in the crystal lattice. Therefore, mullite is an unsaturated, alumina-ordered network structure; its structure has large voids and is relatively loose.

It is the unusual chain structure of mullite that has its C-axis extending into a columnar shape and a needle shape. In refractory materials, these acicular mullites interpenetrate to form a strong skeleton, which has a series of good properties, such as high thermal strength and load softening temperature, good thermal shock resistance, etc. The creep variable is small, the coefficient of linear expansion is small, and the chemical resistance is strong.

The low coefficient of linear expansion of mullite is a typical advantage. Industrially produced mullite generally has a coefficient of linear expansion of about 0.4% at 800 °C. However, the mullite has non-uniformity of axial expansion, and the axial expansion value measured by high temperature XRD indicates that the volume expansion of the solid solution containing Cr2O3 and Fe2O3 and the fused mullite is lower than that of the metered sintered mullite.

In the study of the phase equilibrium of AI2O3-SiO2 system, it has been debated whether mullite is a consistent molten compound. Experiments have shown that mullite is a consistently molten compound when a high purity raw material is used for phase equilibrium under sealed conditions; it is an inconsistent molten compound in the presence of air and impurities. Therefore, it is appropriate to consider mullite as an inconsistent molten compound when analyzing the practical problems of general silicate materials.

The mullite materials used in the refractory industry are generally synthetic and there are few valuable natural mullite deposits. In recent decades, the development and application of synthetic mullite raw materials have been greatly developed. Synthetic mullite is generally produced by sintering or electrofusion.