Mullite is a mineral of long interlocking needle-like crystal structure that is very resistant to thermal shock failure (has a low thermal expansion). It also has a low thermal conductivity and is very refractory thus the theoretical formula (of 71.8% alumina and 28.2% silica) bears little resemblance to the real world material.
On a scale of lowest to highest thermal expansions at 2000F (where fused silica is almost zero and quartz is 1.5%), mullite is about one third of the way. It has a lower expansion than fused alumina (0.9%) and stabilized zircon (0.8%). The resulting bodies display low thermal expansion and a useful in spark plugs, laboratory ware, etc. and in thermal shock resistant refractories.