The Collaborative International Dictionary of English v.0.48:

Feldspar \Feld"spar`\, Feldspath \Feld"spath`\, n. [G.
   feldspath; feld field + spath spar.] (Min.)
   A name given to a group of minerals, closely related in
   crystalline form, and all silicates of alumina with either
   potash, soda, lime, or, in one case, baryta. They occur in
   crystals and crystalline masses, vitreous in luster, and
   breaking rather easily in two directions at right angles to
   each other, or nearly so. The colors are usually white or
   nearly white, flesh-red, bluish, or greenish.
   [1913 Webster]

   Note: The group includes the monoclinic (orthoclastic)
         species orthoclase or common potash feldspar, and the
         rare hyalophane or baryta feldspar; also the triclinic
         species (called in general plagioclase) microcline,
         like orthoclase a potash feldspar; anorthite or lime
         feldspar; albite or soda feldspar; also intermediate
         between the last two species, labradorite, andesine,
         oligoclase, containing both lime and soda in varying
         amounts. The feldspars are essential constituents of
         nearly all crystalline rocks, as granite, gneiss, mica,
         slate, most kinds of basalt and trachyte, etc. The
         decomposition of feldspar has yielded a large part of
         the clay of the soil, also the mineral kaolin, an
         essential material in the making of fine pottery.
         Common feldspar is itself largely used for the same
         purpose. Feldspathic

WordNet (r) 3.0 (2006):

feldspar
    n 1: any of a group of hard crystalline minerals that consist of
         aluminum silicates of potassium or sodium or calcium or
         barium [syn: feldspar, felspar]