The Collaborative International Dictionary of English v.0.48:

Leucite \Leu"cite\ (l[=u]"s[imac]t), n. [Gr. leyko`s white: cf.
   F. leucite.]
   [1913 Webster]
   1. (Min.) A mineral having a glassy fracture, occurring in
      translucent trapezohedral crystals. It is a silicate of
      alumina and potash KAlSi2O6. It is found in the volcanic
      rocks of Italy, especially at Vesuvius.
      [1913 Webster]

   2. (Bot.) A leucoplast.
      [1913 Webster]