Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

orthoclase
     n : a white or colored monoclinic feldspar