The Collaborative International Dictionary of English v.0.48:

Miargyrite \Mi*ar"gy*rite\, n. [Gr. ? less + ? silver. So called
   because it contains less silver than some kindred ore.]
   (Min.)
   A mineral of an iron-black color, and very sectile,
   consisting principally of sulphur, antimony, and silver.
   [1913 Webster]