The Collaborative International Dictionary of English v.0.48:

Arsenopyrite \Ar`sen*o*pyr"ite\, n. [Arsenic + pyrite.] (Min.)
   A mineral of a tin-white color and metallic luster,
   containing arsenic, sulphur, and iron; -- also called
   arsenical pyrites and mispickel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arsenopyrite
    n 1: a silver-white or grey ore of arsenic [syn: arsenopyrite,
         mispickel]