The Collaborative International Dictionary of English v.0.48:

Orthoclase \Or"tho*clase\, n. [Ortho- + Gr. ? to break.] (Min.)
   Common or potash feldspar crystallizing in the monoclinic
   system and having two cleavages at right angles to each
   other. See Feldspar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

orthoclase
    n 1: a white or colored monoclinic feldspar