Talc density is 2.75. Light as far as stone is concerned but heavier than epoxy or cornstarch and much heavier than microballoons. Seems light in a container because it is a fine powder with lots of air between the particles (doesn't compress). Mixed in glue, however, most of the air is expelled and you will have no density improvement but it would likely sand better because it is a very soft mineral.

Edit:
My comments are based upon the physical properties of talc - I have not used it. It is possible that if there is enough entrained air in the mix that the overall density could be low. After all, microballoons are just small spheres full of air with a shell that is quite dense. Maybe someone has some comparative information of the relative densities of real-life mixes.

Ross