Silicon dioxide is powdered silica. As far as I know, it's safe and used in minute amounts to keep powdered supplements from clumping together. Silica is a trace mineral that occurs naturally in herbs like Horsetail. It works together with calcium to strengthen bones, etc. Whole Foods market has a list of "inactive" ingredients and silicon dioxide is listed as GRAS (generally recognized as safe).

By the way, titanium dioxide is not listed as GRAS. I, too, avoid it whenever possible.

Here's the list in case anyone is interested: Inactive Ingredients

Hope this helps,