In reality there occurs a low level of isomorphous substitutions by cations of similar sizes to the Al and Si. Thus Al³⁺ or Fe³⁺ can replace Si⁴⁺ and Mg²⁺ or Fe²⁺ or Fe³⁺ can replace Al³⁺. The differences in charge number can result in significant effects in kaolin properties. If near the crystal face one can see Bronsted acid sites or cation exchange sites. If deeper in the crystal structure background radiation (usually from low levels of radium in the earth’s mantle) can generate electrons or “holes” that can stabilize the lattice charges by being trapped locally. These unpaired charges can be detected by electron spin resonance.