You are an idiot. Silica is an additive for weaponizing anthrax. Find silica in anthrax and you've found weaponization, not "contamination" that doesn't (can't) happen because pure fused silica glass doesn't leave traces of itself (that's why people go to the expense of using it for equipment in lab analysis).
Southie, silica can just blow in if there's lots of finely ground loose silica around. The fact that silica was found can be used to suggest something much more useful like where the envelopes were filled.