﻿374 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  Table 
  II. 
  

   Inversion 
  Points 
  of 
  Tridymite 
  formed 
  under 
  varying 
  conditions. 
  

  

  Temperature 
  Temperature 
  

  

  of 
  inversion 
  of 
  inversion 
  

   Method 
  of 
  formation 
  No. 
  1 
  No. 
  2 
  

  

  No. 
  112. 
  Quartz 
  and 
  Na 
  2 
  W0 
  4 
  heated 
  ) 
  (a) 
  116-2° 
  117 
  . 
  7o 
  164-0° 
  1ft0 
  , 
  

  

  to 
  1000° 
  ± 
  for 
  118 
  hours 
  f 
  (b) 
  119-2° 
  1W 
  ' 
  160-9° 
  1W 
  4 
  

  

  No. 
  106. 
  Quartz 
  and 
  Na 
  2 
  W0 
  4 
  heated 
  ) 
  (a) 
  117-6° 
  11Q>10 
  161-7° 
  1A1 
  , 
  

   to 
  1000° 
  ± 
  for 
  71 
  hours 
  \ 
  (b) 
  118'7° 
  iW 
  L 
  161-2° 
  i0i 
  4 
  

  

  No. 
  144. 
  Quartz 
  and 
  Na 
  2 
  W0 
  4 
  heated 
  ) 
  (a) 
  117-6° 
  117 
  qo 
  165-6° 
  1ft 
  - 
  „ 
  

   to 
  1300° 
  ± 
  for 
  3 
  hours.... 
  J" 
  (b) 
  117-0° 
  iU 
  6 
  164-7° 
  1D0 
  ^ 
  

  

  No. 
  155. 
  Mixture 
  of 
  quartz, 
  tridymite"] 
  

  

  and 
  cristobalite 
  made 
  from 
  amor- 
  ! 
  -m 
  ~ 
  i 
  j 
  1fi9Q0 
  

  

  phous, 
  precipitated 
  Si0 
  2 
  with 
  [ 
  iiri 
  10 
  ^ 
  y 
  

  

  Na 
  2 
  W0 
  4 
  over 
  Bunsen 
  burner 
  J 
  

  

  No. 
  159b. 
  Precipitated 
  Si0 
  2 
  and 
  ) 
  

  

  Na 
  2 
  W0 
  4 
  heated 
  over 
  blast. 
  Some 
  I 
  115-2° 
  162-2° 
  

  

  cristobalite 
  formed 
  at 
  same 
  time 
  ) 
  

  

  No. 
  173. 
  Precipitated 
  Si0 
  2 
  and) 
  iiq.a° 
  1fiof; 
  o 
  

  

  Na 
  2 
  W0 
  4 
  heated 
  41 
  hours 
  at 
  870° 
  ± 
  . 
  . 
  j" 
  li8 
  4 
  lb 
  ^ 
  D 
  

  

  Average 
  _ 
  117-4° 
  - 
  162-8° 
  

  

  stage 
  of 
  a 
  microscope, 
  and 
  the 
  ends 
  of 
  the 
  bar 
  were 
  heated 
  by 
  

   burners. 
  Heating 
  was 
  equalized 
  by 
  covering 
  the 
  bar 
  with 
  

   sheets 
  of 
  asbestos. 
  The 
  approximate 
  temperature 
  was 
  given 
  

   by 
  a 
  thermoelement 
  resting 
  against 
  the 
  bottom 
  of 
  the 
  slide. 
  

   By 
  this 
  means 
  heating 
  and 
  cooling 
  could 
  be 
  conducted 
  rapidly 
  

   or 
  slowly. 
  In 
  addition 
  to 
  observations 
  on 
  cooling 
  phenomena 
  

   a 
  close 
  watch 
  was 
  kept 
  during 
  heating 
  to 
  see 
  if 
  any 
  observable 
  

   change 
  occurred 
  at 
  the 
  second 
  inversion-point. 
  Several 
  experi- 
  

   ments 
  were 
  made, 
  running 
  the 
  temperature 
  up 
  to 
  200° 
  or 
  more 
  

   and 
  then 
  allowing 
  it 
  to 
  drop. 
  The 
  results 
  were 
  practically 
  

   identical 
  in 
  each 
  instance. 
  At 
  a 
  temperature 
  of 
  about 
  120°, 
  

   on 
  heating, 
  the 
  faint 
  birefringence 
  of 
  a 
  basal 
  section 
  suddenly 
  

   disappeared. 
  From 
  this 
  temperature 
  on 
  up 
  to 
  200° 
  (or 
  as 
  far 
  

   as 
  heating 
  was 
  carried) 
  nothing 
  further 
  could 
  be 
  detected 
  

   either 
  on 
  isotropic 
  basal 
  sections 
  or 
  on 
  slightly 
  birefringent 
  

   tilted 
  sections. 
  Upon 
  cooling, 
  the 
  first 
  change 
  noticeable 
  took 
  

   place 
  considerably 
  below 
  120° 
  and 
  was 
  not 
  so 
  abrupt 
  as 
  upon 
  

   heating. 
  At 
  about 
  90° 
  basal 
  sections 
  began 
  to 
  show 
  a 
  little 
  

   birefringence, 
  and 
  at 
  70° 
  to 
  65° 
  the 
  reversion 
  was 
  apparently 
  

   complete. 
  

  

  From 
  these 
  results 
  it 
  is 
  inferred 
  that 
  the 
  optical 
  changes 
  

   attendant 
  upon 
  the 
  second 
  inversion 
  are 
  of 
  a 
  very 
  low 
  order, 
  

   and 
  that 
  reversion 
  upon 
  cooling 
  occurs 
  considerably 
  below 
  the 
  

   point 
  obtained 
  on 
  heating 
  and 
  is 
  spread 
  over 
  a 
  range 
  of 
  tem- 
  

   perature. 
  For 
  that 
  reason 
  no 
  break 
  can 
  be 
  detected 
  upon 
  the 
  

   cooling 
  curve 
  by 
  the 
  methods 
  first 
  employed. 
  

  

  In 
  the 
  absence 
  of 
  direct 
  evidence 
  an 
  opinion 
  upon 
  the 
  sig- 
  

   nificance 
  of 
  the 
  second 
  inversion-point 
  (at 
  163° 
  ca.) 
  is 
  some- 
  

  

  