﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  339 
  

  

  The 
  Appearance 
  of 
  Unstable 
  Phases. 
  

  

  In 
  experiments 
  on 
  the 
  relations 
  of 
  the 
  various 
  forms, 
  certain 
  

   phenomena 
  were 
  met 
  which 
  were 
  rather 
  puzzling 
  at 
  first, 
  but 
  

   after 
  their 
  explanation 
  was 
  perceived 
  it 
  was 
  recognized 
  that 
  

   they 
  threw 
  considerable 
  light 
  upon 
  discrepancies 
  shown 
  in 
  the 
  

   results 
  obtained 
  in 
  previous 
  work 
  upon 
  the 
  silica 
  minerals, 
  and 
  

   upon 
  the 
  conditions 
  under 
  which 
  tridymite 
  and 
  cristobalite 
  

   have 
  been 
  formed 
  in 
  nature. 
  It 
  was 
  found, 
  for 
  example, 
  that 
  

   if 
  silica 
  glass 
  or 
  precipitated 
  silica 
  was 
  heated 
  with 
  sodic 
  tung- 
  

   state 
  for 
  a 
  number 
  of 
  hours 
  at 
  800-850°, 
  not 
  quartz 
  but 
  tridy- 
  

   mite 
  was 
  first 
  obtained. 
  It 
  was 
  only 
  after 
  much 
  longer 
  heat- 
  

   ing 
  that 
  quartz 
  crystals 
  began 
  to 
  appear, 
  although 
  this 
  is 
  the 
  

   stable 
  form 
  at 
  that 
  temperature. 
  It 
  seems 
  that 
  in 
  the 
  passage 
  

   from 
  the 
  amorphous 
  condition 
  to 
  quartz, 
  the 
  whole 
  is 
  first 
  

   converted 
  into 
  the 
  intermediate 
  form 
  tridymite, 
  and 
  only 
  

   secondarily 
  into 
  quartz. 
  

  

  Likewise, 
  if 
  either 
  amorphous 
  modification 
  is 
  heated 
  with- 
  

   out 
  a 
  flux 
  at 
  1300° 
  or 
  1400°, 
  cristobalite 
  alone 
  is 
  obtained, 
  

   although 
  the 
  temperature 
  is 
  within 
  the 
  range 
  of 
  tridymite. 
  

   The 
  process 
  halts 
  at 
  the 
  cristobalite 
  stage, 
  and 
  can 
  only 
  be 
  

   carried 
  to 
  completion 
  by 
  the 
  addition 
  of 
  a 
  flux. 
  Similarly, 
  

   ground 
  quartz 
  heated 
  without 
  a 
  flux 
  at 
  high 
  temperatures 
  but 
  

   still 
  below 
  the 
  1470° 
  inversion-point 
  is 
  changed 
  to 
  cristobalite 
  

   and 
  not 
  tridymite, 
  which 
  might 
  be 
  expected. 
  

  

  A 
  striking 
  instance 
  of 
  the 
  formation 
  of 
  unstable 
  phases 
  

   appeared 
  in 
  a 
  series 
  of 
  experiments 
  in 
  which 
  one 
  or 
  another 
  

   form 
  of 
  silica 
  was 
  heated 
  with 
  a 
  large 
  excess 
  of 
  sodic 
  tungstate 
  

   over 
  a 
  Bunsen 
  burner. 
  In 
  one 
  instance 
  in 
  which 
  amorphous 
  

   precipitated 
  silica 
  was 
  thus 
  heated 
  for 
  43 
  hours, 
  quartz, 
  tridy- 
  

   mite, 
  and 
  cristobalite, 
  all 
  in 
  good 
  crystals, 
  were 
  found 
  in 
  the 
  

   same 
  melt. 
  In 
  other 
  instances, 
  quartz 
  or 
  tridymite 
  or 
  cristo- 
  

   balite 
  was 
  similarly 
  employed 
  and 
  two 
  or 
  three 
  of 
  the 
  phases 
  

   were 
  simultaneously 
  obtained. 
  The 
  crystalline 
  outlines 
  were 
  

   such 
  as 
  to 
  indicate 
  new 
  formation 
  of 
  even 
  that 
  phase 
  which 
  

   was 
  added 
  at 
  the 
  beginning. 
  Working 
  in 
  this 
  manner 
  one 
  

   may 
  start 
  with 
  tridymite 
  and, 
  keeping 
  the 
  temperature 
  in 
  all 
  

   parts 
  of 
  the 
  crucible 
  within 
  the 
  tridymite 
  range, 
  convert 
  part 
  

   of 
  the 
  tridymite 
  into 
  cristobalite— 
  a 
  result 
  which 
  at 
  first 
  sight 
  

   seems 
  impossible. 
  

  

  ^ 
  The 
  production 
  of 
  unstable 
  phases 
  in 
  this 
  manner 
  has 
  con- 
  

   siderable 
  theoretical 
  importance. 
  In 
  its 
  proper 
  interpretation 
  

   light 
  may 
  be 
  thrown 
  upon 
  some 
  apparent 
  discrepancies 
  in 
  

   former 
  work 
  and 
  make 
  possible 
  a 
  reasonable 
  explanation 
  of 
  

   natural 
  occurrences 
  of 
  tridymite 
  and 
  cristobalite. 
  It 
  also 
  

   seems 
  to 
  have 
  some 
  bearing 
  upon 
  recent 
  theories 
  of 
  the 
  

   structure 
  of 
  molecules 
  and 
  crystals. 
  It 
  will, 
  therefore, 
  be 
  dis- 
  

   cussed 
  at 
  some 
  length 
  and 
  an 
  endeavor 
  made 
  to 
  interpret 
  its 
  

   significance. 
  

  

  