The Collaborative International Dictionary of English v.0.48:

Chalybite \Chal"y*bite\, n. (Min.)
   Native iron carbonate; -- usually called siderite.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chalybite
    n 1: iron ore in the form of ferrous carbonate [syn: siderite,
         chalybite]