﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  343 
  

  

  of 
  tridymite 
  and 
  unassociated 
  silica 
  instead 
  of 
  that 
  pertaining 
  

   to 
  cristobalite. 
  , 
  

  

  Thus 
  it 
  seems 
  that 
  whether 
  we 
  regard 
  Ostwald's 
  principle 
  

   from 
  a 
  thermodynamic 
  standpoint, 
  applying 
  the 
  principle 
  of 
  

   minimum 
  free 
  energy, 
  or 
  whether 
  we 
  use 
  a 
  kinetic 
  conception 
  

   of 
  the 
  process 
  as 
  a 
  guide, 
  its 
  validity 
  as 
  a 
  general 
  law 
  is 
  ques- 
  

   tionable. 
  

  

  Although 
  the 
  reactions 
  of 
  silica 
  show 
  a 
  number 
  of 
  phenom- 
  

   ena 
  to 
  which 
  Ostwald's 
  principle 
  is 
  applicable, 
  exceptions 
  are 
  

   also 
  found. 
  Thus 
  if 
  a 
  mixture 
  of 
  amorphous 
  silica 
  and 
  sodic 
  

   tungstate 
  is 
  heated 
  to 
  800-850°, 
  in 
  a 
  few 
  hours 
  only 
  tridymite 
  

   can 
  be 
  found. 
  By 
  much 
  longer 
  heating 
  quartz 
  crystals 
  appear. 
  

   At 
  no 
  stage 
  can 
  cristobalite, 
  the 
  intermediate 
  form 
  between 
  

   amorphous 
  silica 
  and 
  tridymite, 
  be 
  detected. 
  Likewise, 
  silica 
  

   glass 
  or 
  precipitated 
  silica, 
  heated 
  without 
  a 
  flux 
  at 
  any 
  tem- 
  

   perature 
  at 
  which 
  devitrification 
  occurs, 
  always 
  gives 
  cristo- 
  

   balite, 
  but 
  just 
  above 
  1470° 
  it 
  should 
  first 
  give 
  tridymite 
  if 
  

   Ostwald's 
  rule 
  applied. 
  Quartz 
  heated 
  without 
  a 
  flux 
  for 
  a 
  

   very 
  long 
  period 
  at 
  1300° 
  gives 
  cristobalite, 
  in 
  obedience 
  to 
  

   Ostwald's 
  principle. 
  Heated 
  with 
  a 
  flux 
  for 
  three 
  hours 
  at 
  

   the 
  same 
  temperature, 
  it 
  gives 
  tridymite 
  with 
  no 
  indication 
  of 
  

   an 
  intermediate 
  cristobalite 
  phase. 
  

  

  Suggested 
  Explanation 
  of 
  Anomalous 
  Results 
  Previously 
  

   t 
  Obtained. 
  

  

  From 
  the 
  results 
  obtained 
  by 
  former 
  investigators 
  of 
  the 
  

   silica 
  diagram, 
  it 
  had 
  been 
  pretty 
  well 
  established 
  that 
  the 
  

   quartz-tridymite 
  inversion-point 
  lay 
  between 
  800° 
  and 
  900°. 
  

   The 
  exact 
  temperature 
  was 
  rendered 
  uncertain 
  from 
  the 
  fact 
  

   that 
  in 
  a 
  number 
  of 
  cases 
  amorphous 
  silica 
  was 
  employed 
  as 
  

   the 
  initial 
  material, 
  which, 
  as 
  we 
  have 
  seen, 
  in 
  the 
  presence 
  of 
  

   a 
  flux 
  yields 
  tridymite 
  at 
  temperatures 
  considerably 
  below 
  

   the 
  true 
  inversion-point. 
  The 
  significance 
  of 
  this 
  has 
  been 
  

   discussed, 
  and 
  it 
  has 
  been 
  shown 
  that 
  with 
  longer 
  heating 
  

   the 
  tridymite 
  obtained 
  would 
  have 
  gone 
  over 
  into 
  the 
  stable 
  

   form 
  quartz. 
  Similarly 
  misleading 
  phenomena 
  appeared 
  in 
  

   the 
  endeavor 
  to 
  determine 
  the 
  position 
  of 
  cristobalite 
  in 
  the 
  

   series. 
  By 
  heating 
  either 
  quartz 
  or 
  amorphous 
  silica 
  without 
  

   a 
  flux, 
  cristobalite 
  will 
  be 
  obtained 
  at 
  temperatures 
  much 
  below 
  

   its 
  field 
  of 
  stability. 
  

  

  These 
  discordant 
  phenomena 
  may 
  all 
  be 
  interpreted 
  as 
  

   instances 
  in 
  which 
  Ostwald's 
  principle 
  applies, 
  and 
  their 
  

   appearance 
  need 
  give 
  rise 
  to 
  no 
  uncertainty. 
  

  

  Natural 
  Occurrences 
  of 
  Tridymite 
  and 
  Cristobalite. 
  

  

  The 
  mode 
  of 
  occurrence 
  of 
  natural 
  tridymite 
  and 
  cristo- 
  

   balite 
  implies 
  that 
  in 
  many 
  cases 
  they 
  have 
  been 
  formed 
  as 
  

  

  