Mullite rarely occurs as a mineral in nature. In fact, the word mullite is derived from the Isle of Mull off the English coast, where the only naturally occurring deposits of mullite have ever been found. Naturally occurring mullite is so rare because it is the result of extremely high temperatures that have come into contact with aluminosilicate minerals of just the right type.

Despite the fact that mullite rarely occurs in nature it is an extremely valuable mineral to anyone involved in producing products that need to withstand high temperatures, corrosive environments, or other adverse conditions. Its use, therefore, as an industrial mineral, has to be supplied by synthetic alternatives. These methods generally involve high temperature reactions of aluminosilicate minerals such as kyanite, andalusite or sillimanite or alternatively bauxites and kaolins.
Key Properties

Mullite is a very important phase in high temperature, high hot strength, and thermally shock resistant products. It is virtually volume stable at very high temperatures. It has a low coefficient of thermal expansion. It is a good thermal and electrical insulator – even at very high temperatures. It has outstanding hot load-bearing properties, and it is resistant to many corrosive environments. It is, in short, the key ingredient in many refractory and ceramic products