﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  375 
  

  

  what 
  speculative, 
  but 
  a 
  suggestion 
  may 
  be 
  made 
  as 
  to 
  the 
  

   probable 
  nature 
  of 
  the 
  phenomenon. 
  This 
  inversion 
  may 
  

   very 
  well 
  be 
  analogous 
  to 
  that 
  of 
  quartz 
  in 
  passing 
  from 
  the 
  

   a 
  to 
  the 
  IB 
  form. 
  The 
  latter 
  has 
  been 
  supposed 
  to 
  be 
  a 
  change 
  

   from 
  tetartohedral 
  to 
  hemihedral 
  symmetry 
  within 
  the 
  hexag- 
  

   onal 
  system. 
  "With 
  tridymite 
  the 
  change 
  may 
  be 
  one 
  from 
  

   hemihedral 
  hexagonal 
  to 
  holohedral 
  hexagonal. 
  

  

  The 
  low 
  temperature 
  form 
  of 
  tridymite 
  has 
  been 
  called 
  

   a-tridymite, 
  and 
  I 
  have 
  adhered 
  to 
  this 
  usage, 
  although 
  it 
  is 
  

   contrary 
  to 
  the 
  usual 
  custom, 
  which 
  applies 
  the 
  term 
  a 
  to 
  the 
  

   highest 
  form. 
  The 
  term 
  /3-tridymite, 
  it 
  seems 
  now, 
  covers 
  two 
  

   forms, 
  which 
  it 
  is 
  necessary 
  to 
  distinguish. 
  The 
  system 
  of 
  

   nomenclature 
  which 
  is 
  in 
  use 
  is 
  not 
  well 
  adapted 
  to 
  meet 
  such 
  

   a 
  contingency, 
  and 
  I 
  can 
  only 
  suggest 
  that 
  the 
  form 
  stable 
  

   between 
  117° 
  and 
  163° 
  be 
  called 
  /^-tridymite 
  and 
  the 
  form 
  

   stable 
  above 
  163° 
  be 
  called 
  /3 
  2 
  -tridymite. 
  

  

  It 
  was 
  seen 
  in 
  the 
  case 
  of 
  cristobalite 
  that 
  the 
  experimental 
  

   phenomena 
  observed 
  in 
  the 
  inversion 
  from 
  the 
  a 
  to 
  the 
  (3 
  form 
  

   seemed 
  to 
  necessitate 
  a 
  conception 
  of 
  two 
  different 
  sorts 
  of 
  

   silica 
  molecules 
  within 
  the 
  crystal. 
  As 
  a 
  consequence, 
  the 
  tem- 
  

   perature 
  of 
  inversion 
  could 
  be 
  made 
  to 
  vary 
  over 
  a 
  wide 
  range 
  

   by 
  changing 
  the 
  conditions 
  of 
  formation. 
  With 
  tridymite 
  

   nothing 
  of 
  this 
  nature 
  can 
  be 
  detected. 
  The 
  temperatures 
  of 
  

   the 
  two 
  inversions 
  appear 
  to 
  be 
  fixed, 
  whatever 
  the 
  tempera- 
  

   ture 
  of 
  formation 
  and 
  whether 
  quartz 
  or 
  amorphous 
  silica 
  be 
  

   employed 
  at 
  the 
  start. 
  Even 
  under 
  conditions 
  of 
  formation 
  

   of 
  such 
  a 
  nature 
  that 
  quartz, 
  tridymite, 
  and 
  cristobalite 
  are 
  

   simultaneously 
  precipitated 
  from 
  a 
  melt 
  (as 
  in 
  Exper. 
  155, 
  

   Table 
  II), 
  implying 
  the 
  presence 
  of 
  several 
  kinds 
  of 
  molecules 
  

   in 
  solution 
  at 
  the 
  same 
  time, 
  the 
  tridymite 
  obtained 
  gives 
  tem- 
  

   peratures 
  of 
  inversion 
  coinciding 
  with 
  those 
  given 
  by 
  other 
  

   samples. 
  It 
  seems 
  probable, 
  therefore, 
  that 
  the 
  nature 
  and 
  

   distribution 
  of 
  forces 
  within 
  a 
  tridymite 
  crystal 
  are 
  such 
  as 
  to 
  

   admit 
  only 
  one 
  kind 
  of 
  molecule, 
  and 
  in 
  this 
  respect 
  tridymite 
  

   differs 
  from 
  cristobalite. 
  The 
  supposition 
  might 
  be 
  made 
  that 
  

   two 
  kinds 
  of 
  molecules 
  are 
  present 
  in 
  tridymite 
  but 
  that 
  equi- 
  

   librium 
  is 
  maintained 
  by 
  a 
  sufficiently 
  rapid 
  transformation 
  of 
  

   one 
  kind 
  of 
  molecule 
  into 
  another 
  to 
  keep 
  pace 
  with 
  changes 
  

   of 
  temperature. 
  A 
  strong 
  argument 
  against 
  this 
  hypothesis 
  

   is 
  the 
  general 
  reluctance 
  manifested 
  in 
  the 
  transformations 
  of 
  

   silica 
  molecules. 
  

  

  The 
  low-temperature 
  forms 
  of 
  tridymite 
  and 
  cristobalite 
  are 
  

   somewhat 
  unique. 
  Although 
  the 
  reaction 
  a-cristobalite 
  K 
  > 
  

   /3-cristobalite, 
  for 
  instance, 
  is 
  perfectly 
  reversible 
  a-cristobaiite 
  

   has 
  no 
  stable 
  existence 
  and 
  its 
  relation 
  toward 
  quartz 
  is 
  that 
  of 
  

   monotropy. 
  The 
  same 
  is 
  true 
  of 
  a- 
  and 
  /^-tridymite. 
  In 
  this 
  

   respect 
  the 
  reactions 
  of 
  sulphur 
  are 
  quite 
  analogous.* 
  

  

  *R. 
  Brauns, 
  Nenes 
  Jahrb., 
  Beilageband, 
  xxxix, 
  1900. 
  

  

  