A micaceous mineral (hydrated magnesium aluminum silicate).  Usually used in its expanded state.  Heat resistant, absorbent, low density filler.  Not fibrous so it does not add to the physical strength of the product.  Used as a low cost filler, with claims it improves wear characteristics, reduces swell and growth.  The jury is still out on the best particle size to use.  Up to 10% by weight has been used in a formula.