﻿JV. 
  L. 
  Bow 
  en 
  — 
  Sodium-Potassiinn 
  Nephelites. 
  127 
  

  

  The 
  orthorhombic 
  variety 
  is 
  encountered 
  in 
  the 
  mixture 
  

   with 
  20 
  per 
  cent 
  NaAlSi0 
  4 
  only 
  at 
  a 
  considerably 
  higher 
  tem- 
  

   perature, 
  1580°. 
  The 
  inversion 
  point 
  is, 
  therefore, 
  raised, 
  

   which 
  means 
  that 
  the 
  low 
  temperature, 
  hexagonal 
  form 
  takes 
  

   more 
  NaAlSi0 
  4 
  into 
  solid 
  solution 
  than 
  does 
  the 
  high 
  tempera- 
  

   ture 
  form. 
  This 
  is 
  precisely 
  as 
  one 
  would 
  expect 
  it 
  to 
  be, 
  

   since 
  the 
  hexagonal 
  form 
  is 
  so 
  like 
  the 
  corresponding 
  form 
  of 
  

   NaAlSi0 
  4 
  , 
  and, 
  indeed, 
  forms 
  with 
  it 
  an 
  unbroken 
  series 
  of 
  

   mix-crystals. 
  

  

  On 
  the 
  basis 
  of 
  these 
  results 
  fig. 
  2 
  is 
  presented 
  as 
  represent- 
  

   ing 
  a 
  partly 
  hypothetical 
  but 
  very 
  probable 
  equilibrium 
  dia- 
  

   gram 
  of 
  the 
  complete 
  system. 
  Some 
  of 
  the 
  curves 
  are 
  shown 
  

   in 
  broken 
  lines 
  because 
  they 
  are 
  less 
  well 
  supported 
  by 
  ascer- 
  

   tained 
  facts 
  than 
  the 
  others, 
  though 
  they 
  are 
  believed 
  to 
  offer 
  

   the 
  only 
  possible 
  interpretation 
  of 
  the 
  results 
  obtained. 
  

  

  The 
  Composition 
  of 
  Natural 
  Nephelite. 
  

  

  It 
  is 
  now 
  established 
  beyond 
  question 
  that 
  natural 
  nephelites 
  

   are 
  of 
  variable 
  composition. 
  That 
  the 
  molecules 
  NaAlSi0 
  4 
  

   and 
  KAlSi0 
  4 
  are 
  fundamental 
  constituents 
  of 
  nephelite 
  may 
  be 
  

   regarded 
  as 
  definitely 
  decided 
  by 
  the 
  present 
  investigation. 
  

  

  Concerning 
  the 
  excess 
  silica 
  in 
  nephelite 
  above 
  the 
  ortho- 
  

   silicate 
  ratio 
  there 
  is 
  still 
  some 
  controversy. 
  Most 
  investiga- 
  

   tors 
  believe 
  that 
  JN"aAlSiO. 
  and 
  KAlSi0 
  4 
  are 
  fundamental 
  and 
  

  

  4 
  4 
  

  

  that 
  the 
  variable 
  excess 
  of 
  silica 
  is 
  to 
  be 
  ascribed 
  to 
  the 
  pres- 
  

   ence 
  in 
  variable 
  amount 
  of 
  a 
  more 
  silicious 
  molecule. 
  The 
  

   suggestion 
  that 
  this 
  molecule 
  is 
  NaAlSi 
  3 
  8 
  was 
  first 
  made, 
  

   apparently, 
  by 
  Clarke* 
  and 
  later 
  stated 
  in 
  terms 
  of 
  the 
  more 
  

   modern 
  solid 
  solution 
  theory 
  by 
  Schallerf* 
  and 
  the 
  writer.;}; 
  

   Thugutt,§ 
  however, 
  would 
  still 
  assign 
  a 
  definite 
  formula 
  to 
  

   nephelite 
  and 
  assumes 
  that 
  it 
  is 
  

  

  8Na 
  5 
  A1 
  ! 
  Si 
  3 
  O 
  ]0 
  .4Na 
  ! 
  Al,O 
  1 
  .3K 
  ! 
  Al 
  1 
  Si,O 
  10 
  

  

  and 
  that 
  any 
  deviation 
  from 
  this 
  formula 
  is 
  the 
  result 
  of 
  an 
  admix- 
  

   ture 
  of 
  products 
  of 
  its 
  own 
  decomposition, 
  He 
  therein 
  ignores 
  

   the 
  fact 
  that 
  the 
  later 
  students 
  of 
  the 
  composition 
  of 
  nephelite 
  

   have 
  taken 
  the 
  greatest 
  precautions 
  in 
  selecting 
  only 
  absolutely 
  

   fresh 
  material 
  for 
  analysis. 
  |j 
  He 
  likewise 
  fails 
  to 
  consider 
  the 
  fact 
  

   that 
  one 
  can 
  prepare 
  nephelite 
  showing 
  the 
  same 
  variability 
  by 
  

   dry 
  fusion, 
  under 
  conditions 
  absolutely 
  precluding 
  the 
  possibil- 
  

   ity 
  of 
  aqueous 
  decomposition. 
  ^f 
  No 
  consideration 
  is 
  given, 
  

  

  *F. 
  W. 
  Clarke, 
  U. 
  S. 
  Geol. 
  Survey 
  Bull. 
  125, 
  18, 
  1895. 
  

   f 
  Jour. 
  Wash. 
  Acad. 
  Sci., 
  i, 
  109. 
  1911. 
  

   tThis 
  Journal 
  (4), 
  xxxiii, 
  49, 
  1912. 
  

  

  § 
  C. 
  R. 
  Soc. 
  Sci. 
  Varsovie, 
  VI 
  Annee. 
  Fasc. 
  9, 
  862, 
  1913. 
  

   || 
  Morozewicz, 
  Bull. 
  Acad. 
  Sci. 
  Cracovie, 
  958, 
  1907. 
  

  

  if 
  Wallace, 
  Zs. 
  anorg. 
  Chem., 
  lxiii, 
  1, 
  1909 
  ; 
  and 
  Boweu, 
  this 
  Journal 
  (4), 
  

   xxxiii, 
  50, 
  1912. 
  

  

  