What type of minerals are in baseball clay (calcined clay)?

Question

What type of minerals are in baseball clay (calcined clay)?

Answer 1

Mullite

Answer 2

Baseball clay is standard kaolin clay (most common form of clay).  It contains mostly alumina (Al2O3) and silica (SiO2).  This kind of clay is formed naturally starting with the mineral feldspar, where water dissolves away the most soluble parts of the feldspar, leaving the above mentioned minerals, but some water stays bound to the minerals.  Calcining means heating until the bound water is removed.  Since kaolin clay is dug out of the ground in great quantities, there can be trace amounts of many other natural minerals in it but it is mostly alumina and silica.