The Collaborative International Dictionary of English v.0.48:

Sanidine \San"i*dine\, n. [Gr. ?. ?, a board. So called in
   allusion to the tabular crystals.] (Min.)
   A variety of orthoclase feldspar common in certain eruptive
   rocks, as trachyte; -- called also glassy feldspar.
   [1913 Webster]