﻿132 
  A 
  7 
  . 
  L. 
  Bowen 
  — 
  Sodium- 
  Potassium 
  Nephelites. 
  

  

  and 
  carnegieite, 
  with 
  an 
  inversion 
  point 
  at 
  1248°. 
  The 
  high 
  

   temperature 
  form, 
  carnegieite, 
  melts 
  at 
  1526°. 
  The 
  potash 
  

   compound 
  shows 
  two 
  forms, 
  kaliophilite, 
  isomorphous 
  with 
  

   nephelite, 
  and 
  an 
  orthorhombic 
  form 
  with 
  twinning 
  analogous 
  

   to 
  that 
  in 
  aragonite. 
  The 
  orthorhombic 
  form 
  is 
  apparently 
  

   stable 
  at 
  temperatures 
  above 
  1540° 
  and 
  melts 
  in 
  the 
  neighbor- 
  

   hood 
  of 
  1800°. 
  

  

  The 
  potash 
  compound 
  has 
  a 
  eutectic 
  with 
  carnegieite 
  at 
  

   1404°. 
  With 
  nephelite 
  it 
  forms 
  an 
  unbroken 
  series 
  of 
  solid 
  

   solutions. 
  It 
  is 
  concluded, 
  therefore, 
  that 
  NaAlSi0 
  4 
  and 
  

   KAlSi0 
  4 
  are 
  the 
  fundamental 
  molecules 
  of 
  natural 
  nephelites. 
  

   But, 
  in 
  addition 
  to 
  these, 
  nephelites 
  contain 
  variable 
  amounts 
  

   of 
  plagioclase 
  in 
  solid 
  solution, 
  the 
  plagioclase 
  varying 
  from 
  

   albite 
  to 
  anorthite, 
  the 
  latter 
  accounting 
  for 
  the 
  lime 
  content 
  

   and 
  the 
  former 
  for 
  the 
  excess 
  silica 
  of 
  the 
  natural 
  mineral. 
  

   The 
  composition 
  of 
  nephelite 
  should, 
  therefore, 
  be 
  expressed 
  

   in 
  terms 
  of 
  the 
  four 
  molecules 
  NaAlSi0 
  4 
  , 
  KAlSi0 
  4 
  , 
  NaAlSi 
  3 
  8 
  

   and 
  CaAl 
  2 
  Si 
  2 
  8 
  . 
  Reference 
  is 
  made 
  to 
  the 
  petrogenetic 
  

   importance 
  of 
  the 
  occurrence 
  of 
  the 
  last 
  two 
  molecules, 
  viz. 
  

   plagioclase, 
  in 
  nephelites. 
  

  

  Geophysical 
  Laboratory, 
  

  

  Carnegie 
  Institution 
  of 
  Washington, 
  

   Washington, 
  D. 
  C, 
  December 
  4, 
  1916. 
  

  

  