The Collaborative International Dictionary of English v.0.48:

Anorthite \A*nor"thite\, n. [Gr. 'an priv. + 'orqo`s straight
   ('orqh` sc. gwni`a right angle); not in a right angle.]
   A mineral of the feldspar family, commonly occurring in small
   glassy crystals, also a constituent of some igneous rocks. It
   is a lime feldspar. See Feldspar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anorthite
    n 1: rare plagioclastic feldspar occurring in many igneous rocks