﻿346 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  cases 
  which 
  he 
  has 
  observed, 
  such 
  newly 
  formed 
  tridymite 
  

   seems 
  always 
  to 
  be 
  due 
  in 
  some 
  way 
  to 
  the 
  intervention 
  of 
  

   mineralizers, 
  and, 
  as 
  previously 
  shown, 
  the 
  reactions 
  of 
  vapors 
  

   are 
  likely 
  to 
  produce 
  tridymite 
  at 
  a 
  temperature 
  below 
  its 
  range 
  

   of 
  stability. 
  Nevertheless 
  the 
  most 
  probable 
  explanation 
  for 
  

   the 
  phenomena 
  which 
  Lacroix 
  describes 
  seems 
  to 
  be 
  that 
  the 
  

   temperature 
  was 
  sufficiently 
  high 
  to 
  break 
  up 
  the 
  quartz 
  mole- 
  

   cule 
  and 
  give 
  opportunity 
  for 
  the 
  formation 
  of 
  new 
  groupings 
  

   corresponding 
  to 
  tridymite 
  and 
  cristobalite. 
  

  

  A 
  reproduction 
  of 
  the 
  essential 
  conditions 
  attending 
  the 
  

   engulf 
  m 
  en 
  t 
  of 
  quartzose 
  material 
  by 
  a 
  liquid 
  magma 
  was 
  

   attempted 
  in 
  one 
  experiment. 
  Potassium 
  carbonate, 
  sodium 
  

   bicarbonate, 
  and 
  ground 
  basalt 
  were 
  mixed 
  in 
  approximately 
  

   equal 
  proportions, 
  and 
  eight 
  or 
  nine 
  times 
  as 
  much 
  ground 
  

   quartz 
  was 
  added. 
  The 
  whole 
  was 
  heated 
  in 
  a 
  Fletcher 
  fur- 
  

   nace 
  to 
  a 
  high 
  temperature 
  (1500-1600°) 
  and 
  melted 
  to 
  a 
  clear, 
  

   slightly 
  greenish 
  glass, 
  in 
  which 
  small 
  spherulites 
  had 
  formed 
  on 
  

   cooling. 
  This 
  glass 
  was 
  then 
  placed 
  in 
  the 
  electric 
  furnace 
  and 
  

   devitritied 
  by 
  holding 
  at 
  1000° 
  to 
  1400° 
  for 
  five 
  hours. 
  It 
  was 
  

   then 
  found 
  to 
  be 
  filled 
  with 
  a 
  mass 
  of 
  tridymite 
  crystals. 
  A 
  

   portion 
  of 
  this 
  was 
  mixed 
  with 
  a 
  large 
  excess 
  of 
  quartz 
  and 
  the 
  

   whole 
  well 
  ground. 
  Heated 
  again 
  for 
  22 
  hours 
  at 
  1200-1400°, 
  

   the 
  final 
  result 
  was 
  a 
  mixture 
  of 
  tridymite 
  and 
  cristobalite, 
  

   although 
  the 
  temperature 
  appropriate 
  to 
  the 
  cristobalite 
  region 
  

   had 
  never 
  been 
  reached. 
  The 
  results 
  are 
  similar 
  in 
  kind 
  to 
  

   those 
  observed 
  in 
  the 
  basaltic 
  rocks 
  of 
  Mayen,* 
  where 
  quartz 
  

   inclusions 
  have 
  been 
  partly 
  converted 
  into 
  cristobalite 
  and 
  tri- 
  

   dymite, 
  and 
  is 
  to 
  be 
  ascribed 
  to 
  the 
  breaking-up 
  of 
  the 
  quartz 
  

   molecules 
  by 
  the 
  high 
  temperature 
  attained, 
  giving 
  oppor- 
  

   tunity 
  for 
  new 
  arrangements 
  to 
  form, 
  without 
  facilitating 
  

   rearrangement 
  to 
  such 
  a 
  degree 
  that 
  all 
  the 
  groups 
  reached 
  

   stability. 
  

  

  The 
  question 
  arises, 
  whether 
  in 
  the 
  cooling 
  of 
  a 
  magma 
  

   quartz 
  may 
  appear 
  outside 
  of 
  its 
  range 
  of 
  stability, 
  as 
  several 
  

   authors 
  have 
  supposed. 
  The 
  possibility 
  of 
  this 
  is 
  not 
  wholly 
  

   excluded, 
  but 
  nothing 
  has 
  been 
  found 
  in 
  the 
  experimental 
  work 
  

   which 
  suggests 
  anything 
  of 
  the 
  kind, 
  and 
  on 
  theoretical 
  grounds 
  

   it 
  appears 
  inherently 
  improbable. 
  The 
  equilibrium 
  from 
  which 
  

   the 
  magma 
  has 
  cooled 
  is 
  one 
  corresponding 
  to 
  the 
  presence 
  of 
  

   tridymite 
  molecules 
  in 
  the 
  solution. 
  It 
  is 
  difficult 
  to 
  conceive 
  

   the 
  formation 
  and 
  precipitation 
  of 
  quartz 
  from 
  such 
  a 
  solution 
  

   above 
  the 
  inversion 
  -point, 
  while 
  the 
  precipitation 
  of 
  tridy- 
  

   mite 
  below 
  this 
  point 
  is 
  perfectly 
  intelligible 
  when 
  the 
  cooling 
  

   is 
  rapid. 
  The 
  effect 
  of 
  pressure 
  in 
  shifting 
  the 
  inversion-point 
  

   itself 
  is 
  not 
  here 
  considered, 
  but 
  will 
  be 
  taken 
  up 
  later. 
  

  

  * 
  A. 
  Lacroix, 
  Bull. 
  Soc. 
  Min., 
  xiv, 
  185, 
  1891. 
  P. 
  Gaubert, 
  idem, 
  xxvii, 
  42, 
  

   1904. 
  

  

  