﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  357 
  

  

  cristobalite 
  under 
  such 
  conditions. 
  A 
  number 
  of 
  experimenters 
  

   have 
  reported 
  the 
  formation 
  of 
  these 
  two 
  minerals 
  in 
  aqueous 
  

   solution 
  and 
  this 
  fact 
  had, 
  in 
  the 
  beginning, 
  given 
  rise 
  to 
  

   uncertainty 
  in 
  regard 
  to 
  the 
  stability 
  relations 
  of 
  the 
  three. 
  

  

  In 
  all 
  my 
  experiments 
  with 
  amorphous 
  silica 
  in 
  alkaline 
  

   solutions, 
  quartz 
  was 
  obtained. 
  When 
  artificial 
  tridymite 
  or 
  

   cristobalite 
  was 
  substituted 
  for 
  amorphous 
  silica, 
  quartz 
  was 
  

   likewise 
  obtained 
  as 
  the 
  end-product. 
  This 
  removed 
  any 
  

   uncertainty 
  that 
  had 
  been 
  felt 
  as 
  to 
  the 
  relative 
  stability 
  of 
  the 
  

   three 
  minerals 
  under 
  such 
  conditions 
  and 
  confirmed 
  the 
  results 
  

   obtained 
  in 
  tungstate 
  melts. 
  

  

  To 
  obtain 
  as 
  much 
  information 
  as 
  possible 
  on 
  the 
  question, 
  

   it 
  was 
  thought 
  desirable 
  to 
  repeat 
  several 
  of 
  the 
  experiments 
  

   cited 
  in 
  the 
  literature, 
  in 
  which 
  tridymite 
  or 
  cristobalite 
  was 
  

   reported. 
  

  

  E. 
  Baur,"* 
  in 
  one 
  of 
  his 
  experiments 
  (No. 
  8) 
  took 
  5 
  g. 
  Si0 
  2 
  , 
  

   4*3 
  g. 
  Al 
  2 
  Na 
  (composition 
  between 
  soda 
  leucite 
  and 
  nephe- 
  

   lite) 
  and 
  obtained 
  quartz, 
  tridymite, 
  and 
  albite. 
  The 
  tridymite 
  

   was 
  described 
  by 
  Weinschenk 
  as 
  follows 
  : 
  " 
  tablets, 
  made 
  up 
  

   of 
  countless 
  differently 
  oriented 
  individuals, 
  plainly 
  less 
  

   refringent 
  than 
  Canada 
  balsam, 
  weakly 
  birefringent, 
  small 
  

   axial 
  angle, 
  optically 
  positive." 
  To 
  repeat 
  this, 
  I 
  placed 
  in 
  a 
  

   bomb 
  of 
  16 
  cc 
  capacity 
  a 
  thorough 
  mixture 
  of 
  2*5 
  g. 
  amorphous 
  

   precipitated 
  silica 
  and 
  2*15 
  g. 
  NaA10 
  2 
  (the 
  latter 
  made 
  by 
  

   heating 
  a 
  mixture 
  of 
  Na 
  2 
  C0 
  3 
  and 
  A1 
  2 
  3 
  in 
  molecular 
  propor- 
  

   tions 
  to 
  1400°) 
  ; 
  6 
  CC 
  of 
  water 
  was 
  added. 
  The 
  bomb 
  was 
  

   heated 
  to 
  approximately 
  520° 
  for 
  five 
  hours, 
  then 
  heating 
  cur- 
  

   rent 
  was 
  turned 
  off 
  and 
  the 
  bomb 
  cooled 
  with 
  furnace 
  over 
  

   night. 
  The 
  resulting 
  product 
  consisted 
  apparently 
  of 
  two 
  

   different 
  minerals. 
  The 
  first 
  was 
  in 
  sharp, 
  hexagonal 
  prisms 
  

   cut 
  off 
  squarely 
  by 
  basal 
  pinacoid, 
  elongation 
  negative, 
  both 
  

   indices 
  >1*530 
  and 
  <1*535. 
  The 
  crystals 
  are 
  attacked 
  by 
  

   dilute 
  HC1, 
  leaving 
  at 
  times 
  crystalline 
  fragments 
  in 
  an 
  amor- 
  

   phous 
  material 
  (probably 
  gelatinous 
  Si0 
  3 
  ). 
  This 
  conforms 
  to 
  

   nephelite 
  except 
  for 
  slightly 
  lower 
  index. 
  The 
  second 
  mate- 
  

   rial 
  was 
  in 
  roundish 
  granules 
  having 
  at 
  times 
  a 
  suggestion 
  of 
  

   crystal 
  outline, 
  was 
  isotropic 
  and 
  had 
  index 
  just 
  below 
  1*490 
  ; 
  

   apparently 
  analcite. 
  

  

  This 
  experiment 
  was 
  performed 
  three 
  times, 
  with 
  some 
  vari- 
  

   ation 
  as 
  to 
  length 
  of 
  heating 
  and 
  rate 
  of 
  cooling. 
  The 
  prod- 
  

   ucts 
  were 
  always 
  the 
  same. 
  

  

  Although 
  these 
  results 
  do 
  not 
  agree 
  with 
  those 
  obtained, 
  by 
  

   Baur, 
  I 
  do 
  not 
  consider 
  that 
  one 
  disproves 
  the 
  other. 
  There 
  

   can 
  hardly 
  be 
  any 
  question 
  that 
  quartz 
  is 
  the 
  stable 
  mineral 
  

   under 
  these 
  conditions, 
  but 
  it 
  mi^ht 
  well 
  happen 
  that 
  from 
  

   some 
  combination 
  of 
  circumstances 
  the 
  intermediate 
  form 
  

  

  *E. 
  Baur, 
  Zs. 
  phys. 
  Chem., 
  xlii, 
  567-576, 
  1902. 
  

  

  