﻿128 
  JV. 
  L. 
  Bowen 
  — 
  Sodium-Potassium 
  Nejihelites. 
  

  

  moreover, 
  to 
  the 
  fact 
  that 
  NaAlSi0 
  4 
  and 
  mixtures 
  of 
  it 
  with 
  

   KAlSi0 
  4 
  can 
  be 
  prepared 
  in 
  a 
  form 
  identical 
  with 
  nephelite 
  

   and, 
  indeed, 
  occur 
  so 
  in 
  nature 
  in 
  the 
  so-called 
  pseudonephelite, 
  

   whereas 
  the 
  molecules 
  chosen 
  by 
  Thngutt 
  do 
  not 
  occur 
  in 
  

   forms 
  at 
  all 
  suggesting 
  nephelite. 
  

  

  Several 
  objections 
  have 
  been 
  raised 
  to 
  the 
  suggestion 
  that 
  

   the 
  albite 
  molecule, 
  NaAlSi 
  3 
  8 
  , 
  is 
  the 
  siliceous 
  molecule 
  pres- 
  

   ent 
  in 
  nephelite. 
  Thngutt 
  points 
  out 
  that 
  albite 
  has 
  never 
  

   been 
  isolated 
  from 
  nephelite, 
  which 
  is 
  perfectly 
  true, 
  but 
  

   neither 
  has 
  albite 
  been 
  isolated 
  from 
  labradorite. 
  The 
  sugges- 
  

   tion 
  of 
  Foote 
  and 
  Bradley 
  that 
  the 
  question 
  of 
  the 
  condition 
  of 
  

   the 
  excess 
  silica 
  should 
  be 
  left 
  open* 
  is 
  safe, 
  to 
  be 
  sure, 
  but 
  

   there 
  are 
  nevertheless 
  good 
  reasons 
  for 
  preferring 
  the 
  albite 
  

   molecule. 
  

  

  It 
  was 
  formerly 
  considered, 
  if 
  a 
  triclinic 
  mineral 
  was 
  taken 
  

   into 
  solution 
  by 
  an 
  hexagonal 
  mineral, 
  that 
  the 
  triclinic 
  mineral 
  

   must 
  be 
  dimorphous, 
  must 
  have 
  an 
  hexagonal 
  modification. 
  

   But 
  this 
  idea 
  is 
  now 
  known 
  to 
  be 
  contrary 
  to 
  well-ascertained 
  

   facts. 
  We 
  may 
  take 
  the 
  case 
  of 
  anorthite 
  in 
  solid 
  solution 
  in 
  

   nephelite, 
  which 
  extends 
  as 
  far 
  as 
  35 
  per 
  cent 
  anorthite, 
  yet 
  

   anorthite 
  has 
  no 
  hexagonal 
  modification. 
  It 
  seems 
  to 
  the 
  

   writer, 
  in 
  the 
  light 
  of 
  recent 
  studies 
  of 
  the 
  actual 
  atomic 
  

   structure 
  of 
  crystals, 
  to 
  be 
  more 
  reasonable 
  to 
  assume 
  that 
  the 
  

   anorthite 
  atomic 
  pattern, 
  though 
  of 
  triclinic 
  symmetry, 
  is 
  

   nevertheless 
  sufficiently 
  close 
  to 
  the 
  hexagonal 
  symmetry 
  of 
  

   nephelite 
  or 
  sufficiently 
  amenable 
  to 
  modification 
  that, 
  under 
  

   the 
  influence 
  of 
  the 
  fields 
  of 
  force 
  existing 
  within 
  a 
  nephelite 
  

   crystal, 
  it 
  is 
  capable 
  of 
  being 
  so 
  modified 
  as 
  to 
  conform 
  with 
  

   the 
  hexagonal 
  pattern, 
  though 
  not 
  in 
  unlimited 
  amounts. 
  Is 
  it 
  

   surprising, 
  then, 
  that 
  the 
  albite 
  atomic 
  pattern 
  should 
  be 
  

   capable 
  of 
  precisely 
  the 
  same 
  modification 
  by 
  a 
  nephelite 
  

   crystal, 
  up 
  to 
  a 
  certain 
  limit, 
  when 
  we 
  consider 
  the 
  fact 
  that 
  

   albite 
  and 
  anorthite 
  are 
  so 
  nearly 
  alike 
  in 
  pattern 
  that 
  they 
  

   form 
  an 
  unbroken 
  series 
  of 
  mix-crystals 
  ? 
  

  

  Or 
  if 
  we 
  look 
  at 
  the 
  question 
  from 
  the 
  point 
  of 
  view 
  of 
  

   phase 
  equilibria 
  and 
  remember 
  that 
  albite 
  and 
  anorthite 
  in 
  any 
  

   system 
  form 
  not 
  two 
  phases 
  but 
  the 
  single 
  phase, 
  plagioclase, 
  

   then 
  it 
  becomes 
  quite 
  inappropriate 
  to 
  speak 
  of 
  the 
  solubility 
  

   in 
  nephelite 
  of 
  anorthite 
  and 
  of 
  albite 
  except 
  as 
  limiting 
  

   values 
  of 
  the 
  solubilities 
  of 
  the 
  phase, 
  plagioclase. 
  Any 
  

   nephelite 
  forming 
  in 
  a 
  magma 
  together 
  with 
  plagioclase 
  must, 
  

   of 
  necessity, 
  dissolve 
  the 
  amount 
  of 
  both 
  anorthite 
  and 
  albite 
  

   (i. 
  e. 
  of 
  the 
  phase 
  plagioclase) 
  that 
  is 
  required 
  by 
  the 
  condi- 
  

   tions 
  prevailing, 
  sufficient 
  opportunity 
  for 
  equilibrium 
  to 
  

   become 
  established 
  being 
  assumed. 
  

  

  It 
  may 
  be 
  said, 
  then, 
  that 
  theoretical 
  considerations 
  afford 
  

  

  * 
  This 
  Journal 
  (4). 
  xxxiii, 
  439, 
  1912. 
  

  

  