That's fine, but it should be *tested* to see if pure anthrax spores, with silica contamination dumped on top, suddenly aerosolize readily after going through mail processing.
My own opinion is that weaponization of anthrax takes a bit more thorough mixing (at an entirely different molecular level) than that, but why guess when we could test and KNOW?!
Also, keep in mind that it is highly unusual for contamination to be from only one compound/element. Your clothes will have more than one natural element on them after a day of wear outside, after all.
After the anthrax attack they retrofited to use vacuum systems. Most of the aerosolization occurred when anthrax contamination on surfaces was hit with blasts of air at high pressure.
You would not believe......
BTW, I strongly suspect Ken had, in fact, "tested" everything you mention. But that was back, that was back, that was back in the USSR.