All Minerals

Mullite

Formula: Al6Si2O13

Mullite (3Al₂O₃·2SiO₂) is a naturally occurring aluminum silicate crystalline mineral. It forms in high-temperature metamorphic environments. But it is rare in nature and not commercially mined. Mullite is manufactured as a powder by calcining high-alumina + silica materials. It is sold as fused mullite, sintered mullite and mullite grog.

Although the material is not added as a powder to stoneware, porcelain, or refractories, if you are producing them you are still working with it because it forms in situ as needle-shaped crystals growing within the body matrix (even if it is glassy). It forms directly from the kaolin particles (as smaller primary crystals) and later from the feldspar melt within the body as long interlocking needles (secondary mullite). At full development, bodies are more than just a bunch of silica particles glued together in a feldspar-silicate glass, the mullite creates a fibrous mesh.

Mullite is not only largely responsible for the fired strength of vitrified bodies, but also extra structural rigidity during vitrification (pyroplastic pieces are less likely to collapse in the kiln). Mullite offers an extra bonus to fired bodies: Lower thermal expansion.

In ceramic science texts, mullite appears in the Al₂O₃ - SiO₂ phase diagram (the only stable intermediate compound in that binary system).

In refractories, fused mullite aggregate is added (as grog or sand), to impart high-temperature structural stability and avoid creep. High alumina kiln shelves are a major beneficiary of mullite properties.

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