The Collaborative International Dictionary of English v.0.48:

Castor \Cas"tor\, Castorite \Cas"tor*ite\, n. [The minerals
   castor and pollux were so named because found together on the
   island of Elba. See Castor and Pollux.] (Min.)
   A variety of the mineral called petalite, from Elba.
   [1913 Webster]