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  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
  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. 
  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