Silica is a common drying agent. One of the suggestions for the presence of silica in the Daschle anthrax is that silica was used as some kind of primitive drying agent - and traces remained. Silica is also a common additive to keep things from sticking together - not as a coating but as a separator. Here's one description from HERE.
Series-SD silica additive is mainly used as the anti-blocking agent, thickener, stabilizer and carrier in the area of the medicine, food and forage, also as the dryness absorbent of the essence and spicy. Its appearance is uniformly ivory powder. Due to the large specific surface area and the sub-microcosmic of the silica, it can separate with other powder substance, at the same time it promote the freedom fluidness resulting in the improvement of the wet absorption and anti-block ability.
So, to repeat, to say that an additive is a coating is total nonsense.
Ed