Theoretical FORMULA for clay: Al2O3 + 2SiO2 + 2H2O
Clay derives from the decomposition of granite rock, which is composed of alumina, silica and flux.
The deposited alumina and silica particles combined with water that formed the pure clay. Clay contains physical water, which evaporates during drying and firing.
However, this is an ideal formula. Most clay contains some sort of impurities materials, these variations create the different characteristic of each clay type.