The Collaborative International Dictionary of English v.0.48:

Plagionite \Pla"gi*o*nite\, n. [Gr. pla`gios oblique. So called
   in allusion to its usually oblique crystallization.] (Min.)
   A sulphide of lead and antimony, of a blackish lead-gray
   color and metallic luster.
   [1913 Webster]