﻿MAY 
  4, 
  1922 
  MOREY 
  : 
  CRYSTALIvIZATlON 
  PRESSURE 
  IN 
  MAGMAS 
  225 
  

  

  attended 
  by 
  sudden 
  liberation 
  of 
  vapor, 
  taking 
  place 
  as 
  a 
  result 
  of 
  

   cooling. 
  

  

  A 
  further 
  example, 
  illustrating, 
  from 
  the 
  experimental 
  results, 
  

   the 
  development 
  of 
  a 
  fairly 
  high 
  pressure 
  in 
  a 
  silicate 
  system 
  as 
  the 
  

   result 
  of 
  cooling, 
  may 
  be 
  found 
  in 
  the 
  same 
  system. 
  The 
  eutectic 
  

   between 
  K2Si205 
  and 
  vSiOs 
  lies 
  at 
  the 
  remarkably 
  low 
  temperature 
  of 
  

   520°. 
  If 
  a 
  mixture 
  of 
  KoO, 
  SiOo 
  and 
  HoO, 
  containing 
  9.1 
  per 
  cent 
  of 
  

   HoO, 
  with 
  the 
  other 
  ingredients 
  in 
  the 
  molecular 
  ratio 
  Si02/K20 
  = 
  

   4.26, 
  be 
  cooled 
  from 
  a 
  high 
  temperature, 
  the 
  vapor 
  pressure 
  of 
  the 
  mix- 
  

   ture 
  ■v\'ill 
  fall 
  as 
  the 
  temperature 
  falls. 
  The 
  mixture 
  will 
  not 
  begin 
  

   to 
  freeze 
  until 
  it 
  has 
  cooled 
  to 
  500°, 
  when 
  crystals 
  of 
  quartz 
  and 
  the 
  

   ternary 
  compound 
  KHSi205 
  will 
  separate. 
  The 
  vapor 
  pressure 
  of 
  the 
  

   solution 
  at 
  this 
  temperature 
  is 
  160 
  atmospheres. 
  On 
  further 
  cooling, 
  

   the 
  substances 
  continue 
  to 
  crystallize 
  and 
  the 
  pressure 
  increases 
  rap- 
  

   idly. 
  When 
  the 
  temperature 
  has 
  fallen 
  20 
  °, 
  to 
  480 
  °, 
  the 
  water 
  content 
  

   has 
  increased 
  to 
  10.2 
  per 
  cent, 
  and 
  the 
  pressure 
  to 
  180 
  atmospheres. 
  

   When 
  the 
  temperature 
  has 
  fallen 
  to 
  420°, 
  the 
  water 
  content 
  has 
  in- 
  

   creased 
  to 
  12.5 
  per 
  cent, 
  and 
  the 
  pressure 
  to 
  340 
  atmospheres, 
  more 
  

   than 
  double 
  the 
  pressure 
  at 
  500°. 
  

  

  In 
  discussing 
  the 
  crystallization 
  of 
  this 
  mixture, 
  it 
  was 
  assumed 
  that 
  

   crystallization 
  started 
  at 
  500°, 
  the 
  saturation 
  temperature 
  of 
  the 
  mix- 
  

   ture, 
  and 
  the 
  pressure 
  rose 
  from 
  160 
  atmospheres 
  to 
  340 
  atmospheres 
  

   continuously 
  as 
  the 
  mixture 
  crystallized 
  on 
  cooling. 
  It 
  is 
  of 
  interest 
  to 
  

   consider 
  what 
  would 
  happen 
  if 
  the 
  mixture 
  were 
  to 
  cool 
  without 
  crystal 
  - 
  

   lizing, 
  say 
  to 
  420°, 
  and 
  then 
  begin 
  to 
  crystallize. 
  In 
  figure 
  1, 
  B, 
  the 
  

   curve 
  ay, 
  giving 
  the 
  change 
  in 
  pressure 
  with 
  temperature 
  of 
  the 
  mixture 
  

   of 
  composition 
  y 
  in 
  figure 
  1, 
  C, 
  is 
  shown 
  cutting 
  the 
  curve 
  KBm 
  at 
  a 
  

   sharp 
  angle; 
  its 
  metas 
  table 
  prolongation, 
  shown 
  broken 
  in 
  the 
  

   figure, 
  gives 
  the 
  change 
  in 
  vapor 
  pressure 
  that 
  would 
  take 
  place 
  if 
  the 
  

   mixture 
  failed 
  to 
  crystallize. 
  It 
  is 
  safe 
  to 
  assume 
  that 
  this 
  curve 
  has 
  a 
  

   steep 
  slope. 
  Similarly, 
  if 
  the 
  mixture 
  in 
  the 
  ternary 
  system 
  were 
  to 
  

   supercool, 
  its 
  pressure 
  would 
  diminish 
  rapidly, 
  and 
  the 
  amount 
  of 
  

   the 
  diminution 
  can 
  be 
  estimated. 
  The 
  vapor 
  phase 
  consists 
  of 
  water 
  

   only 
  at 
  a 
  pressure 
  of 
  160 
  atmospheres. 
  Pure 
  water 
  has 
  a 
  vapor 
  pres- 
  

   sure 
  of 
  160 
  atmospheres 
  at 
  348°, 
  and 
  it 
  is 
  probable 
  that 
  the 
  two 
  vapor- 
  

   pressure 
  curves 
  will 
  be 
  roughly 
  parallel, 
  with 
  the 
  curve 
  of 
  the 
  saturated 
  

   solution 
  possibly 
  falling 
  more 
  rapidly 
  than 
  that 
  of 
  pure 
  water. 
  On 
  the 
  

   assumption 
  that 
  the 
  drop 
  in 
  pressure 
  for 
  the 
  80 
  ° 
  drop 
  in 
  temperature 
  

   from 
  500 
  to 
  420 
  ° 
  in 
  the 
  solution 
  is 
  the 
  same 
  as 
  the 
  drop 
  in 
  pressure 
  of 
  

   water 
  from 
  348° 
  to 
  a 
  temperature 
  80° 
  lower, 
  the 
  vapor 
  pressure 
  of 
  

  

  