Calcined kaolin (also called metakaolin) is a powdered white non-plastic material. It is raw kaolin that has been fired (in a rotary calcining kiln) high enough to remove the 12% (approx) crystal water. The material is a good example of how we can alter the mineralogy of a material to affect its working properties while maintaining the chemistry to maintain fired properties.
Calcination makes the powder whiter and more chemically inert. That makes it useful in a wide variety of products and industries. But strangely, it is often under-utilized in traditional ceramics . Kaolin is pure clay mineral, having a fired chemistry of 1 part Al2O3 and 2 parts SiO2. But the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Al2O3 is essential to the chemistry of the vast majority of glazes and kaolin is an ideal source (because all glazes also need the SiO2 that it supplies, and, it readily decomposes in the melt).