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Mullite [Al₆Si₂O₁₃ = 3Al₂O₃·2SiO₂], with 71.8 wt.% Al₂O₃ ,  is an important silicate mineral that occurs in high-silica alumina refractories. Mullite exhibits a high melting point of 1810°C combined with low thermal expansion coefficients (i.e., 4.5 × 10^–6 K^–1 parallel to the a-axis and 5.7 × 10^–6 K^–1 parallel to the c-axis), a good mechanical strength with a tensile strength of 62 MPa, and resilience at elevated temperatures that make mullite a highly suitable mineral for highly refractory materials. In nature, mullite is an extremely scarce mineral that occurs only in melted argillaceous inclusions entrapped in lavas from the Cenozoic Era on the Island of Mull, Scotland, but no deposit was found to be economically minable.

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colorless; rhomb, a=0.7557nm, b=0.76876 nm, c=0.28842 nm; hardness: hot pressed 13.6GPa, sintered 12.7GPa; indentation microfracture 2.02MPa ·m1/2; submicrometer powder(s) can be prepared by hydrolysis of mixed alkoxides, followed by drying and calcining up to 1600°C; sinter, microstructure [SOM91]; phases equilibria in mullite [PAS88]; other data in [HIR89] [ROB67] [MIZ89]

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Mullite is present in the form of needles in porcelain, it is used in high-mullite based products is in hot blast stove checker bricks, melting and firing processes, and is used as a refractory material, manufacture of a type of crucible, steel industry, electronic substrates and protective coatings.

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Artificial mullite, or synthetic mullite, a ceramic material made by a prolonged fusing in the electric furnace of a mixture of silica sand or diasphoric clay and bauxite, has the composition 3Al₂O₃·2SiO₂, has a melting point of 1810 C, and softens at 1650 C.
Mullite occurs in nearly all ceramic products containing alumina and silica but, with the exception of refractories, is seldom introduced as such except as calcined kyanite.
The bricks are resistant to flame and to molten ash, and have a low, uniform coefficient of thermal expansion and a heat conductivity only slightly above that of fireclays. Normally, mullite has very fine crystals that change form and become enlarged after prolonged heating, making the product porous and permeable. For stable high-temperature refractories the mullite is prefused to produce larger crystals. At very high temperatures mullite tends to decompose to form corundum and alkali-silicate minerals of lower heat resistance. Mullite is also used for making spark plugs, chemical crucibles, and extruding dies, and a foamed mullite is used as a uniformly latticed honeycomb structure for lightweight, heat-resistant structural parts.

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