The Collaborative International Dictionary of English v.0.48:

Natrolite \Na"tro*lite\ (?; 277), n. [Natron + -lite: cf. F.
   natrolithe.] (Min.)
   A zeolite occuring in groups of glassy acicular crystals, and
   in masses which often have a radiated structure. It is a
   hydrous silicate of alumina and soda.
   [1913 Webster]

WordNet (r) 3.0 (2006):

natrolite
    n 1: a group of minerals of the zeolite family consisting of a
         hydrous silicate of sodium and aluminum