﻿344 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  unstable 
  phases. 
  Cristobalite 
  especially 
  could 
  seldom 
  if 
  ever 
  

   have 
  been 
  deposited 
  as 
  a 
  stable 
  mineral 
  in 
  the 
  circumstances 
  in 
  

   which 
  it 
  is 
  now 
  found. 
  Its 
  usual 
  occurrence 
  is 
  in 
  cavities 
  in 
  

   eruptive 
  rocks, 
  which 
  would 
  certainly 
  have 
  been 
  in 
  a 
  fluid 
  con- 
  

   dition 
  at 
  temperatures 
  within 
  the 
  range 
  of 
  stability 
  of 
  this 
  

   mineral. 
  The 
  appearance 
  of 
  the 
  inclosing 
  rocks 
  in 
  many 
  cases, 
  

   however, 
  suggests 
  that 
  the 
  formation 
  of 
  the 
  cristobalite 
  is 
  to 
  be 
  

   ascribed 
  to 
  its 
  deposition 
  from 
  mutually 
  reacting 
  vapors 
  or 
  to 
  

   the 
  decomposition 
  of 
  former 
  silicate 
  minerals 
  or 
  siliceous 
  glass 
  

   by 
  pneumatolytic 
  processes. 
  In 
  either 
  of 
  such 
  modes 
  of 
  action, 
  

   unassociated 
  molecules 
  of 
  silica 
  would 
  be 
  set 
  free 
  in 
  quantity 
  

   from 
  their 
  previous 
  state 
  of 
  combination 
  with 
  other 
  elements, 
  

   and 
  would 
  probably 
  tend 
  to 
  form 
  groups 
  among 
  themselves 
  

   corresponding 
  to 
  cristobalite. 
  This 
  is 
  perfectly 
  analogous 
  in 
  

   principle 
  to 
  the 
  formation 
  of 
  cristobalite 
  in 
  tungstate 
  melts 
  

   over 
  a 
  Bunsen 
  flame 
  at 
  a 
  comparatively 
  low 
  temperature, 
  which 
  

   was 
  obtained 
  as 
  a 
  result 
  of 
  direct 
  experiment 
  and 
  for 
  which 
  

   the 
  explanation 
  has 
  been 
  suggested. 
  It 
  is 
  quite 
  certain 
  that 
  

   for 
  the 
  deposition 
  of 
  the 
  mineral 
  in 
  question 
  under 
  such 
  condi- 
  

   tions 
  no 
  excessively 
  high 
  temperature 
  is 
  demanded 
  and 
  its 
  

   presence 
  in 
  no 
  wise 
  implies 
  that 
  the 
  temperature 
  requirements 
  

   of 
  stability 
  obtained. 
  

  

  Many 
  of 
  the 
  occurrences 
  of 
  tridymite 
  are 
  similar 
  and 
  like- 
  

   wise 
  suggest 
  the 
  intervention 
  of 
  gases 
  in 
  its 
  production, 
  as 
  has 
  

   been 
  pointed 
  out 
  in 
  a 
  number 
  of 
  instances 
  by 
  A. 
  Lacroix.* 
  An 
  

   interesting 
  association 
  of 
  tridymite 
  and 
  quartz 
  in 
  hollow 
  spheru- 
  

   lites 
  of 
  rhyolite 
  is 
  described 
  by 
  Iddings 
  and 
  Penfield,f 
  and 
  

   appears 
  to 
  be 
  due 
  to 
  processes 
  of 
  this 
  character. 
  The 
  presence 
  

   of 
  trapezohedral 
  faces 
  on 
  the 
  quartz 
  crystals 
  shows 
  that 
  it 
  is 
  of 
  

   the 
  a 
  variety, 
  and 
  therefore 
  formed 
  below 
  575°. 
  

  

  In 
  addition 
  to 
  such 
  occurrences, 
  which 
  point 
  to 
  pneumato- 
  

   lytic 
  action, 
  tridymite 
  is 
  sometimes 
  found 
  as 
  an 
  essential 
  con- 
  

   stituent 
  of 
  acid 
  effusives, 
  associated 
  in 
  such 
  manner 
  with 
  

   other 
  minerals 
  as 
  to 
  imply 
  its 
  separation 
  from 
  the 
  melt 
  as 
  a 
  

   primary 
  constituent. 
  In 
  such 
  instances 
  the 
  implication 
  is 
  

   simply 
  that 
  at 
  some 
  period 
  in 
  the 
  previous 
  history 
  of 
  the 
  

   magma 
  Ihe 
  temperature 
  was 
  such 
  that 
  the 
  excess 
  of 
  silica 
  not 
  

   required 
  to 
  form 
  other 
  minerals 
  had 
  formed 
  the 
  molecular 
  

   groupings 
  corresponding 
  to 
  tridymite, 
  and 
  when 
  rapid 
  cooling 
  

   ensued 
  these 
  groups 
  crystallized 
  out 
  in 
  the 
  tridymite 
  form. 
  

   The 
  temperature 
  at 
  the 
  time 
  of 
  crystallization 
  may 
  have 
  been 
  

   either 
  above 
  or 
  below 
  the 
  870° 
  inversion 
  point. 
  If 
  below, 
  the 
  

   viscosity 
  of 
  the 
  melt 
  acted 
  as 
  an 
  effective 
  obstacle 
  to 
  prevent 
  

   that 
  rearrangement 
  of 
  the 
  molecules 
  which 
  would 
  be 
  demanded 
  

   to 
  form 
  quartz. 
  With 
  less 
  rapid 
  cooling 
  and 
  especially 
  with 
  

  

  * 
  A. 
  Lacroix, 
  Bull. 
  Soc, 
  Min., 
  xxviii, 
  56, 
  1905. 
  

  

  + 
  Iddings 
  and 
  Penfield, 
  this 
  Journal, 
  (3), 
  xlii, 
  39, 
  1891. 
  

  

  