We start with clay, dug from the earth. While I'm certain that the manufacturers are using materials that have been beneficiated (to some degree, "purified"), there's still bound to be some differences in the mineral contents of the raw materials.

When the truck- or railcar loads of clay arrive at the factory, they are blended with a solution known as a "binder" that allows the clay particles to stick together during the pelletization process, and to give the pellets the strength to stay intact during handling in their "green", or unfired state. Those binders can be made up of any number of chemicals, with some of the more common ones being based upon methylcellulose and polyvinyl chloride, but often with sodium salt components. I have even seen situations where diesel fuel was used as the binder. This is an important thing to keep in mind, as if the ceramic body is fired properly, all of the organic components of the raw materials will burn out, while inorganic materials will not, and if the firing is not done correctly, not only is it difficult to know exactly what is still in the ceramic body, but chemical reactions may have occurred that result in less-than-desirable residues (I have gotten bags of a common brand of South American LECA that still reeked of the fuel oil used as the binder).

After firing, the pellets exit the kiln, but are still too hot to handle and package, so it is common to "quench" them quickly by spraying water on them (which also helps control the dust).

At that point, we have the fired, finished-product - clay pellets in a bag - ready to find it's way to the grower. However, those clay pellets may have some odd residues from the firing process, and I think it's safe to say that pure water was not used to quench them as they exited the kiln, so who knows what the water may have added. It is for that reason that it is important to thoroughly wash the pellets before use.

I will not claim any knowledge of the actual makeup of the residues - it's bound to vary all over the map anyway - but I will share my cleaning technique, which I know works:

Sometimes, if I know I will be potting up plants "poorly timed" for the process, I'll go through another overnight soak of clear water with rooting hormone solution.