Source : Webster's Revised Unabridged Dictionary (1913)

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.