﻿366 
  Fenner— 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  temperature 
  has 
  been 
  reached 
  ; 
  or 
  it 
  may 
  involve 
  a 
  more 
  radi- 
  

   cal 
  change 
  in 
  the 
  structure, 
  due 
  to 
  the 
  formation 
  of 
  a 
  new 
  

   type 
  of 
  molecule. 
  All 
  these 
  possibilities 
  seem 
  to 
  be 
  illustrated 
  

   by 
  one 
  or 
  another 
  of 
  the 
  silica 
  transformations, 
  a- 
  and 
  /3-cris- 
  

   tobalite 
  apparently 
  furnish 
  examples 
  of 
  crystal 
  species 
  consist- 
  

   ing 
  each 
  of 
  two 
  different 
  kinds 
  of 
  molecules, 
  and 
  the 
  inversion 
  

   is 
  due 
  to 
  an 
  unmixing 
  in 
  the 
  sense 
  which 
  Smits 
  conceives. 
  

   Quartz 
  and 
  tridymite, 
  on 
  the 
  other 
  hand, 
  seem 
  to 
  consist 
  each 
  

  

  Fig. 
  7. 
  

  

  f\ 
  

  

  if 
  k* 
  

  

  U 
  

   (L 
  

  

  D 
  

   H 
  

   < 
  

  

  or 
  

   u 
  

   Q. 
  

  

  hi 
  

  

  .c£ 
  

  

  s<£ 
  

  

  &£* 
  

  

  *3> 
  

  

  & 
  

  

  6^ 
  

  

  K! 
  

  

  K' 
  

  

  COMPOSITION 
  

  

  of 
  one 
  kind 
  of 
  molecule 
  only. 
  Their 
  low 
  temperature 
  inver- 
  

   sions 
  seem 
  to 
  be 
  due 
  merely 
  to 
  a 
  slight 
  modification 
  of 
  the 
  

   crystal 
  space-lattice, 
  while 
  the 
  transformation 
  of 
  one 
  into 
  

   another 
  at 
  870° 
  seems 
  to 
  imply 
  a 
  more 
  radical 
  change, 
  that 
  of 
  

   one 
  species 
  of 
  molecules 
  into 
  another, 
  and 
  secondarily 
  a 
  change 
  

   of 
  crystal-structure. 
  

  

  The 
  phenomena 
  exhibited 
  in 
  the 
  a-/3 
  inversions 
  of 
  cristo- 
  

   balite 
  are 
  of 
  the 
  following 
  general 
  nature: 
  When 
  the 
  material 
  

  

  