﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  383 
  

  

  the 
  charge. 
  The 
  furnace 
  having 
  been 
  brought 
  to 
  a 
  constant 
  

   high 
  temperature 
  the 
  bucket 
  of 
  quartz 
  was 
  introduced 
  and 
  

   maintained 
  at 
  the 
  same 
  temperature 
  for 
  30 
  minutes 
  to 
  1 
  hour. 
  

   After 
  exposure 
  to 
  1530° 
  for 
  30 
  minutes 
  the 
  mass 
  was 
  tightly 
  sin- 
  

   tered. 
  Microscopic 
  examination 
  showed 
  the 
  quartz 
  grains 
  to 
  

   be 
  bordered 
  by 
  a 
  band 
  of 
  appreciable 
  width, 
  whose 
  index 
  was 
  

   generally 
  less 
  than 
  1*475 
  and 
  in 
  some 
  instances 
  less 
  than 
  1-465 
  ; 
  

   it 
  was 
  isotropic 
  to 
  faintly 
  birefringent 
  ; 
  and 
  consisted 
  apparently 
  

   of 
  a 
  mixture 
  of 
  glass 
  (n=l*460) 
  and 
  cristobalite 
  (n=l'485). 
  

   After 
  exposure 
  to 
  1470° 
  for 
  one 
  hour 
  in 
  another 
  experiment 
  

   the 
  mass 
  was 
  likewise 
  tightly 
  caked. 
  When 
  immersed 
  in 
  a 
  

   liquid 
  of 
  index 
  1*485 
  many 
  of 
  the 
  quartz 
  grains 
  were 
  seen 
  to 
  

   be 
  surrounded 
  by 
  a 
  narrow 
  border 
  of 
  distinctly 
  lower 
  index. 
  

   This 
  same 
  material 
  also 
  cut 
  across 
  many 
  grains, 
  leaving 
  

   detached 
  portions 
  of 
  crystals, 
  and 
  caused 
  an 
  irregular 
  pitting 
  

   and 
  corrosion 
  of 
  others. 
  It 
  was 
  found 
  to 
  be 
  generally 
  faintly 
  

   birefringent 
  and 
  was 
  probably 
  a 
  mixture 
  of 
  glass 
  and 
  cristo- 
  

   balite. 
  That 
  it 
  was 
  not 
  entirely 
  cristobalite 
  was 
  shown 
  by 
  the 
  

   fact 
  that 
  it 
  could 
  be 
  discerned 
  in 
  liquids 
  of 
  successively 
  lower 
  

   index 
  — 
  1*480, 
  1*475, 
  and 
  1*470. 
  In 
  liquid 
  1*465 
  it 
  appeared 
  in 
  

   most 
  cases 
  to 
  have 
  a 
  higher 
  index 
  than 
  the 
  liquid, 
  but 
  several 
  

   good 
  cases 
  were 
  found 
  in 
  which 
  the 
  index 
  was 
  lower 
  and 
  the 
  

   material 
  was 
  isotropic. 
  It 
  appears, 
  therefore, 
  that 
  the 
  fusing- 
  

   point 
  of 
  quartz 
  is 
  lower 
  than 
  1470°, 
  but 
  that 
  at 
  this 
  tempera- 
  

   ture 
  it 
  passes 
  into 
  cristobalite 
  almost 
  as 
  rapidly 
  as 
  it 
  melts. 
  

  

  The 
  fusing-point 
  of 
  tridymite 
  should 
  lie 
  between 
  those 
  of 
  

   quartz 
  and 
  cristobalite. 
  The 
  artificial 
  material 
  obtained 
  from 
  

   tungstate 
  melts 
  always 
  retains 
  a 
  small 
  amount 
  of 
  the 
  flux, 
  

   which 
  would 
  affect 
  the 
  melting-point, 
  and 
  no 
  attempt 
  was 
  

   made 
  to 
  determine 
  it. 
  

  

  Summary. 
  

  

  The 
  relations 
  between 
  the 
  mineral 
  species 
  quartz, 
  tridymite, 
  

   and 
  cristobalite 
  have 
  been 
  found 
  to 
  be 
  enantiotropic. 
  The 
  

   inversion-temperatures 
  under 
  atmospheric 
  pressure 
  are 
  

  

  870° 
  ±10° 
  quartz 
  ^ 
  — 
  > 
  tridymite 
  

   1470° 
  ± 
  10° 
  tridymite 
  ^Z± 
  cristobalite. 
  

  

  The 
  velocity 
  of 
  transformation 
  of 
  one 
  form 
  of 
  silica 
  into 
  

   another 
  has 
  been 
  found 
  to 
  be 
  very 
  slow 
  and 
  in 
  many 
  cases 
  the 
  

   process 
  follows 
  Ostwald's 
  rule 
  ; 
  that 
  is, 
  an 
  unstable 
  form 
  does 
  

   not 
  pass 
  directly 
  into 
  the 
  most 
  stable 
  form, 
  but 
  the 
  action 
  pro- 
  

   gresses 
  through 
  successive 
  steps, 
  and 
  intermediate 
  phases 
  

   appear, 
  which 
  eventually 
  reach 
  the 
  stage 
  of 
  greatest 
  stability. 
  

   The 
  appearance 
  of 
  unstable 
  phases 
  in 
  this 
  manner 
  has 
  sug- 
  

   gested 
  an 
  explanation 
  of 
  natural 
  occurrences 
  of 
  tridymite 
  and 
  

   cristobalite 
  under 
  such 
  conditions 
  as 
  preclude 
  the 
  idea 
  of 
  their 
  

  

  