The Collaborative International Dictionary of English v.0.48:

Anorthosite \A*nor"tho*site\, n. [F. anorthose triclinic
   feldspar (fr. Gr. 'an priv. + 'orqo`s straight) + -ite.]
   (Petrol.)
   A granular igneous rock composed almost exclusively of a
   soda-lime feldspar, usually labradorite.
   [Webster 1913 Suppl.]