﻿498 
  NEW 
  YORK 
  STATE 
  MTJSEUM 
  

  

  The 
  orthoclase 
  or 
  potash 
  feldspar 
  has 
  a 
  composition 
  of 
  silica 
  

   64.7, 
  alumina 
  18.4, 
  potash 
  16.9, 
  while 
  in 
  the 
  plagioclase 
  group 
  the 
  

   composition 
  of 
  the 
  different 
  members 
  is 
  given 
  as 
  follows.-^ 
  

  

  Silica 
  A-lumina 
  Potash 
  Soda 
  Lime 
  

  

  Albite 
  68 
  20 
  ... 
  12 
  12 
  

  

  Oligoclase 
  62 
  24 
  ... 
  9 
  5 
  

  

  Labradorite 
  53 
  30 
  ... 
  ,4 
  13 
  

  

  Anorthite 
  43 
  .37 
  ... 
  '. 
  . 
  . 
  20 
  

  

  In 
  treating 
  the 
  decomposition 
  or 
  kaolinization 
  of 
  feldspar, 
  most 
  

   writers 
  are 
  apt 
  tO' 
  give 
  the 
  impression 
  that 
  it 
  is 
  the 
  orthoclase 
  which 
  

   furnishes 
  kaolinite 
  bj 
  its 
  decomposition, 
  whereas 
  both 
  groups 
  may 
  

   produce 
  it, 
  and 
  indeed 
  the 
  plagioclase 
  varieties 
  decompose 
  much 
  

   more 
  readily 
  than 
  the 
  orthoclase. 
  This 
  fact 
  was 
  noted 
  by 
  Leim- 
  

   berg. 
  (Z. 
  d. 
  d. 
  G. 
  G. 
  35, 
  1883) 
  The 
  same 
  fact 
  was 
  observed 
  by 
  the 
  

   writer 
  in 
  the 
  kaolin 
  at 
  Ronne, 
  Denmark, 
  which 
  is 
  produced 
  by 
  

   the 
  decomposition 
  of 
  a 
  granite 
  containing 
  both 
  plagioclase 
  and 
  

   orthoclase. 
  In 
  partially 
  weathered 
  specimens 
  the 
  plagioclase 
  was 
  

   the 
  more 
  extensively 
  affected. 
  As 
  a 
  rule 
  the 
  orthoclase 
  feldspar 
  is 
  

   much 
  more 
  common 
  than 
  the 
  plagioclase. 
  

  

  Aside 
  from 
  the 
  kaolinization 
  of 
  feldspar 
  by 
  the 
  ordinary 
  processes 
  

   of 
  weathering 
  it 
  seems 
  possible 
  and 
  even 
  probable^ 
  that 
  its 
  decom- 
  

   position 
  may 
  be 
  brought 
  about 
  by 
  the 
  action 
  of 
  mineralizing 
  vapors, 
  

   that 
  is, 
  vapors 
  whose 
  presence 
  seems 
  to 
  be 
  necessary 
  to 
  the 
  forma- 
  

   tion 
  of 
  certain 
  minerals, 
  as 
  at 
  Cornwall, 
  Eng., 
  where 
  it 
  was 
  found 
  

   that 
  the 
  feldspar 
  of 
  the 
  granite 
  on 
  both 
  sides 
  of 
  the 
  tin 
  veins 
  had 
  

   been 
  altered 
  to 
  kaolin. 
  This 
  change 
  is 
  attributed 
  to 
  the 
  action 
  of 
  

   fluoric 
  vapors 
  whose 
  presence 
  is 
  pretty 
  well 
  proven. 
  That 
  such 
  

   a 
  process 
  is 
  possible 
  is 
  shown 
  by 
  J. 
  H. 
  Collins 
  {Min. 
  mag.- 
  1887. 
  

   7:213, 
  in 
  the 
  ^^ 
  !^ature 
  and 
  origin 
  of 
  xilays 
  and 
  the 
  composition 
  of 
  

   kaolinite 
  ") 
  who 
  exposed 
  feldspar 
  to 
  the 
  action 
  of 
  hydrofluoric 
  acid. 
  

   The 
  feldspar, 
  according 
  to 
  Mr 
  Collins, 
  was 
  converted 
  into 
  hydrated 
  

   silicate 
  of 
  alumina, 
  mixed 
  with 
  soluble 
  fluorid 
  of 
  potassium, 
  while 
  

   pure 
  silica 
  was 
  deposited 
  on 
  the 
  sides 
  of 
  the 
  tube. 
  

  

  ' 
  G. 
  P. 
  Merrill. 
  Rocks, 
  rock-weathering 
  and 
  soils, 
  p. 
  15. 
  

  

  