The Collaborative International Dictionary of English v.0.48:

Plagioclase \Pla"gi*o*clase\, n. [Gr. pla`gios oblique + kla^n
   to break.] (Min.)
   A general term used of any triclinic feldspar. See the Note
   under Feldspar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plagioclase
    n 1: any of a series of triclinic feldspars that form rocks
         [syn: plagioclase, oligoclase]