a silvery-white, hard, brittle, monoclinic mineral, FeAsS; iron arsenic sulfide: it is an ore of arsenic found as a twinned crystal in veins of metamorphic rock
See arsenopyrite in American Heritage Dictionary 4
(ärˌsə-nō-pīˈrīt)
noun
A silver-white to gray arsenic ore, essentially FeAsS. Also called mispickel.