The Collaborative International Dictionary
Plagionite
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.
Wiktionary
plagionite
n. (context mineralogy English) A monoclinic-prismatic mineral containing antimony, lead, and sulfur.