orthoclase

from WordNet (r) 3.0 (2006)

orthoclase
    n 1: a white or colored monoclinic feldspar

from 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]