Thanks for the links. I can't believe I'd never heard of bentonite before Sept 11th. It seems to be very available.
Bentonite sucks for this purpose. It is comparitively large grained and dense. If it was present at all it was use to cut the good stuff in the first 2 letters to the news guys. The good stuff that was cut used colloidal silica as the carrier. It has an extremely low density and small particle size. Note Dachle's letter had the good uncut stuff made with silica.
If you ever poured kitty litter...the dust settles fast. If you've ever handled colloidal silica, you'd notice it doesn't settle out of the air and it gets all over the place with the slightest of air movement.