From Natura:
"French green healing clay (also a smectite). Also, a strong absorbent of mineral, including heavy metals & other impurities & toxins preventing them from being absorbed."
http://www.dinovite.com/mont.html:
Montmorillonite clay is a trace mineral-rich clay with some very interesting properties, to help promote your dog’s good health. First, montmorillonite clay is a colloid that is an electrically charged particle. Colloids naturally bind to toxins in your dog’s digestive system, effectively removing them from your dog’s system via his stool. This action helps dogs with bad allergies by binding to and removing the offenders from your dogs system. Colloidal minerals are also readily assimilated by your dog’s system.
I couldn't find anything from AAFCO.