The Collaborative International Dictionary of English v.0.48:

Pollucite \Pol"lu*cite\, n. [See Pollux, and 4th Castor.]
   (Min.)
   A colorless transparent mineral, resembling quartz, occurring
   with castor or castorite on the island of Elba. It is a
   silicate of alumina and c[ae]sia. Called also pollux.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pollucite
    n 1: a rare mineral that is an important source of cesium