Webster's English Dictionary

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feldspar

feld.spar also feld.spath.ose \'fel(d)-.spa:r\ \fel(d)-'spath-ik\ 
   \'fel(d)-.spath-.o-s\ n [modif. of obs. G feldspath (now feldspat), fr. G 
   feld field]+ obs. G spath (now spat) spar : any of a group of crystalline 
   minerals that consist of aluminum silicate s with either potassium, sodium, 
   calcium, or barium and that are an essential constituent of nearly all 
   crystalline rocks (hardness 6-6.5, sp. gr. 2.5-2.9) - feld.spath.ic aj