﻿384 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  deposition 
  within 
  their 
  range 
  of 
  stability, 
  and 
  an 
  inquiry 
  has 
  

   been 
  directed 
  toward 
  the 
  circumstances 
  attending 
  their 
  forma- 
  

   tion. 
  

  

  The 
  preparations 
  of 
  artificial 
  tridymite 
  and 
  cristobalite 
  

   which 
  have 
  been 
  made 
  have 
  yielded 
  the 
  minerals 
  in 
  very 
  pure 
  

   form 
  and 
  a 
  re-determination 
  has 
  been 
  made 
  of 
  a 
  number 
  of 
  

   their 
  optical 
  and 
  other 
  physical 
  constants. 
  The 
  relations 
  of 
  

   chalcedony 
  have 
  been 
  investigated 
  and 
  evidence 
  has 
  been 
  

   obtained 
  tending 
  to 
  show 
  that 
  it 
  is 
  a 
  distinct 
  mineral 
  species. 
  

  

  New 
  determinations 
  of 
  the 
  a-/5 
  inversions 
  of 
  the 
  several 
  

   species 
  have 
  given 
  the 
  following 
  results 
  : 
  

  

  j 
  a-quartz 
  > 
  /3-quartz 
  575° 
  

  

  ( 
  /3-quartz 
  > 
  a-quartz 
  570° 
  

  

  a- 
  tridymite 
  > 
  /^-tridymite 
  117° 
  

  

  /3, 
  -tridymite 
  > 
  /3 
  2 
  -tridymite 
  163° 
  

  

  Reversions 
  on 
  cooling 
  not 
  very 
  sharp 
  

  

  f 
  a-cristobalite 
  >/3-cristobalite 
  274*6° 
  to 
  

  

  219-7°, 
  depending 
  upon 
  previous 
  heat- 
  

   J 
  treatment 
  

  

  j 
  /3-cristobalite 
  > 
  a-cristobalite 
  240*5° 
  to 
  

  

  198-1°, 
  depending 
  upon 
  previous 
  heat- 
  

   [ 
  treatment 
  

   A 
  study 
  of 
  the 
  remarkable 
  variations 
  in 
  the 
  temperature 
  of 
  

   inversion 
  of 
  a- 
  into 
  /3-cristobalite 
  has 
  led 
  to 
  the 
  conclusion 
  that 
  

   this 
  mineral 
  is 
  made 
  up 
  of 
  two 
  different 
  molecular 
  species 
  of 
  

   silica 
  within 
  the 
  same 
  crystal. 
  Various 
  other 
  properties 
  of 
  the 
  

   silica 
  minerals 
  seem 
  to 
  have 
  considerable 
  bearing 
  upon 
  theories 
  

   of 
  the 
  structure 
  of 
  molecules 
  and 
  crystals. 
  The 
  nature 
  of 
  the 
  

   radical 
  differences 
  existing 
  between 
  the 
  two 
  different 
  types 
  of 
  

   inversion 
  has 
  been 
  discussed 
  in 
  some 
  detail. 
  

  

  The 
  melting-point 
  of 
  cristobalite 
  has 
  been 
  found 
  to 
  be 
  close 
  

   to 
  1625°. 
  Quartz 
  melts 
  at 
  least 
  155° 
  lower. 
  

  

  The 
  general 
  stability 
  relations 
  are 
  shown 
  diagrammatically 
  in 
  

   figure 
  1. 
  

  

  Geophysical 
  Laboratory, 
  

  

  Carnegie 
  Institution 
  of 
  Washington, 
  

   Washington, 
  July, 
  1913. 
  

  

  