vermiculite
    n 1: any of a group of yellow or brown hydrous silicate minerals
         having a micaceous structure

Vermiculite \Ver*mic"u*lite\, n. [L. vermiculus, dim. of vermis
   worm.] (Min.)
   A group of minerals having, a micaceous structure. They are
   hydrous silicates, derived generally from the alteration of
   some kind of mica. So called because the scales, when heated,
   open out into wormlike forms.
   [1913 Webster] Vermiculose