﻿JV. 
  L. 
  Bow 
  en 
  — 
  Sodium- 
  Potassiitm 
  Nejphelites. 
  115 
  

  

  Akt. 
  XII. 
  — 
  The 
  Sodium- 
  Potassium 
  JSfejohelites 
  ; 
  by 
  

  

  N. 
  L. 
  Bowen. 
  

  

  Introduction. 
  

  

  Among 
  rock-forming 
  minerals, 
  especially 
  those 
  of 
  the 
  inter- 
  

   esting 
  alkaline 
  rocks, 
  neplielite 
  takes 
  a 
  prominent 
  place 
  and 
  for 
  

   this 
  reason 
  has 
  attracted 
  considerable 
  study 
  on 
  the 
  part 
  of 
  

   mineralogists 
  and 
  petrologists. 
  This 
  study 
  has 
  revealed 
  in 
  

   nephelite 
  considerable 
  chemical 
  complexity 
  the 
  explanation 
  of 
  

   which 
  has 
  been 
  and, 
  to 
  some 
  extent, 
  still 
  is 
  a 
  matter 
  of 
  con- 
  

   troversy. 
  The 
  simple 
  compound 
  NaAlSi0 
  4 
  is 
  now 
  rather 
  

   generally 
  regarded 
  as 
  the 
  fundamental 
  molecule 
  of 
  nephelite, 
  

   but 
  the 
  composition 
  of 
  the 
  natural 
  mineral 
  always 
  departs 
  from 
  

   this 
  markedly, 
  showing 
  a 
  large 
  content 
  of 
  potash, 
  often 
  con- 
  

   siderable 
  lime, 
  and 
  a 
  variable 
  excess 
  of 
  silica 
  above 
  the 
  ortho- 
  

   silicate 
  ratio. 
  A 
  study 
  of 
  the 
  fundamental 
  compound 
  and 
  of 
  

   the 
  lime 
  content 
  of 
  nephelite 
  has 
  already 
  been 
  made 
  at 
  this 
  

   laboratory.* 
  The 
  present 
  paper 
  describes 
  the 
  continuance 
  of 
  

   this 
  work 
  in 
  the 
  study 
  of 
  the 
  potash-bearing 
  nephelites 
  or 
  the 
  

   binary 
  system, 
  NaAlSi0 
  4 
  -KAlSi0 
  4 
  . 
  

  

  The 
  End 
  Members. 
  

  

  JVaAlSiO^. 
  — 
  The 
  compound 
  NaAlSi0 
  4 
  can 
  be 
  prepared 
  by 
  

   sintering 
  together 
  Na 
  2 
  C0 
  3 
  , 
  A1 
  2 
  3 
  and 
  Si0 
  2 
  in 
  the 
  proper 
  pro- 
  

   portions 
  at 
  a 
  low 
  temperature 
  (about 
  800°) 
  to 
  prevent 
  loss 
  of 
  

   soda. 
  Sintering 
  must 
  be 
  repeated 
  several 
  times 
  with 
  inter- 
  

   mediate 
  grindings 
  and 
  then 
  the 
  whole 
  may 
  be 
  raised 
  above 
  the 
  

   melting 
  temperature. 
  The 
  product 
  is 
  a 
  clear 
  homogeneous 
  

   glass 
  of 
  the 
  appropriate 
  composition. 
  f 
  This 
  glass 
  crystallizes 
  

   at 
  low 
  temperatures 
  to 
  the 
  hexagonal 
  form, 
  nephelite, 
  and 
  at 
  

   high 
  temperatures 
  to 
  the 
  triclinic 
  (?) 
  form, 
  carnegieite. 
  The 
  

   inversion 
  temperature 
  of 
  1248° 
  ± 
  5 
  obtained 
  in 
  the 
  earlier 
  

   work 
  was 
  confirmed 
  during 
  the 
  present 
  investigation, 
  as 
  was 
  

   also 
  the 
  melting 
  temperature 
  of 
  carnegiete, 
  1526°. 
  

  

  Pure 
  sodium 
  nephelite 
  is 
  hexagonal 
  and 
  negative. 
  The 
  re- 
  

   fractive 
  indices 
  are 
  t» 
  = 
  1-537 
  ± 
  '002 
  ; 
  e 
  = 
  1-533 
  =fc 
  -002. 
  The 
  

   density 
  is 
  2-619 
  at 
  21°. 
  

  

  Carnegieite 
  always 
  shows 
  a 
  complicated 
  polysynthetic 
  twin- 
  

   ning. 
  There 
  are 
  often 
  two 
  intersecting 
  sets 
  of 
  lamellae 
  sug- 
  

   gesting 
  the 
  microcline 
  twinning 
  and 
  again 
  three 
  sets 
  giving 
  an 
  

   hexagonal 
  aspect. 
  It 
  has 
  been 
  considered 
  triclinic, 
  though 
  it 
  

   could 
  be, 
  perhaps, 
  orthorhombic 
  or 
  monoclinic. 
  The 
  refractive 
  

  

  * 
  N. 
  L. 
  Bowen, 
  this 
  Journal, 
  xxxiii, 
  551, 
  1912. 
  

  

  fThe 
  reasons 
  for 
  this 
  procedure 
  are 
  given 
  in 
  the 
  former 
  paper, 
  this 
  

   Journal, 
  xxxiii, 
  552, 
  1912. 
  

  

  