﻿358 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  tridymite 
  was 
  first 
  produced 
  and 
  from 
  lack 
  of 
  time 
  did 
  not 
  

   pass 
  over 
  completely 
  into 
  quartz. 
  

  

  K. 
  v. 
  Chrustschofi,* 
  by 
  heating 
  soluble 
  amorphous 
  silicic 
  

   acid 
  in 
  an 
  aqueous 
  solution 
  of 
  hydronuoboric 
  acid 
  for 
  five 
  

   hours, 
  obtained 
  the 
  following 
  results 
  : 
  at 
  180-228°, 
  regular 
  

   crystals, 
  perfectly 
  isotropic, 
  index 
  = 
  1*58 
  (possibly 
  a 
  misprint 
  

   for 
  1-48), 
  contain 
  99*78 
  per 
  cent 
  Si0 
  2 
  ; 
  240-300°: 
  quartz; 
  

   310-360° 
  : 
  tridymite 
  with 
  some 
  quartz. 
  

  

  The 
  regular 
  crystals 
  were 
  considered 
  to 
  be 
  cristobalite, 
  

   although 
  the 
  index 
  as 
  quoted 
  is 
  markedly 
  different. 
  No 
  data 
  

   on 
  the 
  tridymite 
  are 
  given 
  in 
  the 
  German 
  abstract. 
  

  

  The 
  writer 
  placed 
  in 
  a 
  bomb 
  of 
  18 
  cc 
  capacity 
  4g. 
  amor- 
  

   phous 
  precipitated 
  silica, 
  3 
  CC 
  hydronuoboric 
  acid, 
  made 
  by 
  

   saturating 
  a 
  40 
  per 
  cent 
  solution 
  of 
  HF 
  with 
  B 
  2 
  3 
  ; 
  and 
  3 
  CC 
  

   water. 
  These 
  were 
  heated 
  22 
  hours 
  at 
  350-380°. 
  The 
  

   product 
  was 
  mostly 
  unchanged 
  amorphous 
  silica, 
  with 
  which 
  

   there 
  were 
  a 
  few 
  small 
  but 
  perfectly 
  formed 
  crystals 
  of 
  quartz. 
  

  

  Cristobalite 
  and 
  especially 
  tridymite 
  have 
  been 
  reported 
  as 
  

   formed 
  similarly 
  in 
  a 
  wet 
  way 
  in 
  numerous 
  instances 
  and 
  

   natural 
  occurrences 
  due 
  to 
  a 
  similar 
  mode 
  of 
  formation 
  have 
  

   likewise 
  been 
  reported. 
  There 
  is 
  no 
  reason 
  known 
  why 
  they 
  

   should 
  not 
  have 
  been 
  deposited 
  as 
  unstable 
  forms 
  under 
  such 
  

   conditions, 
  but 
  these 
  two 
  minerals 
  possess 
  such 
  neutral 
  proper- 
  

   ties 
  that 
  great 
  care 
  must 
  be 
  exercised 
  in 
  identification 
  and 
  

   other 
  possibilities 
  must 
  be 
  eliminated 
  before 
  reaching 
  a 
  positive 
  

   conclusion 
  in 
  such 
  instances. 
  

  

  General 
  Observations 
  on 
  the 
  Quartz-Tridymite-Cristobal- 
  

   ite 
  Inversions. 
  

  

  The 
  experimental 
  work 
  which 
  has 
  thus 
  far 
  been 
  described 
  

   has 
  been 
  concerned 
  principally 
  with 
  establishing 
  the 
  stability 
  

   relations 
  of 
  the 
  three 
  minerals. 
  During 
  the 
  course 
  of 
  the 
  in- 
  

   vestigation, 
  however, 
  a 
  considerable 
  amount 
  of 
  data 
  was 
  accumu- 
  

   lated 
  regarding 
  the 
  conditions 
  under 
  which 
  one 
  form 
  may 
  be 
  

   converted 
  into 
  another 
  regardless 
  of 
  whether 
  the 
  product 
  was 
  

   the 
  final 
  or 
  stable 
  form, 
  and 
  regarding 
  the 
  reactions 
  which 
  may 
  

   be 
  expected 
  under 
  various 
  conditions 
  of 
  treatment. 
  Some 
  of 
  

   these 
  results 
  are 
  important 
  in 
  establishing 
  points 
  in 
  the 
  unstable 
  

   fields 
  of 
  the 
  complete 
  silica 
  diagram. 
  The 
  whole 
  may 
  be 
  sum- 
  

   marized 
  as 
  follows 
  : 
  — 
  

  

  Neither 
  quartz 
  nor 
  tridymite 
  has 
  been 
  formed 
  under 
  any 
  

   conditions 
  in 
  the 
  absence 
  of 
  a 
  solvent. 
  

  

  At 
  temperatures 
  below 
  870° 
  quartz 
  was 
  always 
  produced 
  

   when 
  any 
  form 
  of 
  silica 
  was 
  heated 
  for 
  a 
  sufficient 
  length 
  of 
  

   time 
  in 
  a 
  sodic 
  tungstate 
  melt 
  or 
  in 
  aqueous 
  solution. 
  In 
  a 
  

   sodic 
  tungstate 
  melt 
  the 
  most 
  favorable 
  temperature 
  seemed 
  to 
  

  

  *K. 
  v. 
  Chrustschoff, 
  Nenes 
  Jahrb., 
  i, 
  Eeferate 
  240, 
  1897. 
  

  

  