The Collaborative International Dictionary of English v.0.48:

Chalcanthite \Chal*can"thite\, n. [L. chalcanthum a solution of
   blue vitriol, Gr. ?.] (Min.)
   Native blue vitriol. See Blue vitriol, under Blue.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chalcanthite
    n 1: hydrated blue crystalline form of copper sulfate [syn:
         blue vitriol, blue copperas, blue stone,
         chalcanthite]