﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  379 
  

  

  material 
  in 
  solid 
  solution. 
  The 
  presence 
  of 
  a 
  second 
  molecular 
  

   species 
  of 
  silica 
  in 
  the 
  crystals 
  is 
  a 
  possibility, 
  but 
  the 
  normal 
  

   behavior 
  of 
  other 
  quartz 
  from 
  the 
  same 
  locality 
  is 
  opposed 
  to 
  

   this 
  explanalion. 
  

  

  Quartz 
  in 
  Pegmatites. 
  — 
  The 
  criteria 
  for 
  distinguishing 
  quartz 
  

   formed 
  above 
  575° 
  from 
  that 
  formed 
  below 
  this 
  temperature, 
  

   which 
  Wright 
  and 
  Larsen 
  have 
  developed, 
  have 
  enabled 
  these 
  

   writers 
  and 
  E. 
  S. 
  Bastin* 
  to 
  show 
  that 
  the 
  quartz 
  of 
  pegmatite 
  

   veins 
  has 
  frequently 
  been 
  formed 
  in 
  the 
  neighborhood 
  of 
  the 
  575° 
  

   inversion-point. 
  This 
  may 
  be, 
  in 
  part, 
  merely 
  a 
  coincidence, 
  

   but 
  it 
  may 
  be 
  suggested 
  that 
  the 
  contraction 
  of 
  the 
  quartz 
  in 
  

   siliceous 
  masses 
  when 
  cooling 
  through 
  the 
  inversion 
  temperature 
  

   may 
  be 
  a 
  contributing 
  factor. 
  Recent 
  unpublished 
  work 
  by 
  

   R. 
  B. 
  Sosman 
  in 
  this 
  Laboratory 
  has 
  shown 
  that 
  in 
  cooling 
  

   from 
  600° 
  to 
  550° 
  the 
  volume 
  of 
  quartz 
  decreases 
  about 
  2 
  per 
  

   cent. 
  This 
  rather 
  sudden 
  contraction 
  being 
  superposed 
  upon 
  

   the 
  normal 
  contraction 
  which 
  a 
  mass 
  of 
  quartziferous 
  igneous 
  

   rock 
  undergoes 
  in 
  cooling 
  will 
  favor 
  the 
  development 
  of 
  fissures 
  

   and 
  their 
  subsequent 
  filling 
  at 
  about 
  the 
  inversion 
  temperature. 
  

  

  Two 
  types 
  of 
  Silica 
  Inversions. 
  — 
  We 
  have 
  seen 
  that 
  the 
  

   inversions 
  of 
  silica 
  belong 
  to 
  two 
  radically 
  different 
  types. 
  One 
  

   is 
  distinguished 
  by 
  a 
  small 
  energy-change, 
  small 
  change 
  of 
  

   optical 
  and 
  crystallographic 
  properties, 
  and 
  readiness 
  of 
  reac- 
  

   tion 
  ; 
  the 
  other 
  by 
  a 
  much 
  greater 
  change 
  of 
  optical 
  and 
  crystal- 
  

   lographic 
  properties, 
  by 
  sluggishness 
  of 
  reaction, 
  and 
  by 
  energy- 
  

   changes 
  probably 
  of 
  considerable 
  amount. 
  The 
  evidence 
  which 
  

   has 
  been 
  presented 
  suggests 
  an 
  explanation 
  of 
  the 
  differences. 
  

   It 
  points 
  to 
  the 
  conclusion 
  that 
  in 
  inversions 
  of 
  the 
  first 
  type 
  

   the 
  process 
  is 
  simply 
  a 
  small 
  rearrangement 
  of 
  the 
  molecules 
  

   within 
  the 
  crystal-structure 
  ; 
  in 
  those 
  of 
  the 
  second 
  type 
  the 
  

   change 
  is 
  much 
  more 
  radical 
  and 
  involves 
  the 
  destruction 
  of 
  

   one 
  sort 
  of 
  molecular 
  species 
  and 
  the 
  formation 
  of 
  another. 
  

  

  Relations 
  of 
  Chalcedony 
  to 
  other 
  Forms 
  of 
  Silica. 
  

  

  Evidence 
  on 
  the 
  relations 
  of 
  chalcedony 
  to 
  quartz 
  has 
  been 
  

   mostly 
  of 
  an 
  optical 
  character. 
  The 
  difficulties 
  in 
  the 
  determina- 
  

   tion 
  of 
  the 
  optical 
  properties 
  of 
  fibrous 
  chalcedony 
  have 
  been 
  so 
  

   great 
  that 
  no 
  really 
  conclusive 
  proof 
  of 
  identity 
  or 
  lack 
  of 
  

   identity 
  with 
  quartz 
  has 
  been 
  forthcoming. 
  Arguments 
  on 
  

   both 
  sides 
  have 
  been 
  presented 
  and 
  at 
  the 
  present 
  time 
  the 
  

   question 
  seems 
  to 
  be 
  largely 
  a 
  matter 
  of 
  personal 
  opinion 
  among 
  

   those 
  mineralogists 
  who 
  adhere 
  to 
  one 
  side 
  or 
  the 
  other. 
  Some 
  

   thermal 
  work 
  has 
  been 
  done 
  by 
  Le 
  Chatelier 
  and 
  the 
  results 
  

   are 
  frequently 
  quoted, 
  but 
  careful 
  study 
  shows 
  that 
  they 
  are 
  

  

  * 
  Wright 
  and 
  Larsen, 
  op. 
  cit., 
  pp. 
  446-447. 
  E. 
  S. 
  Bastin, 
  Jour. 
  Geol., 
  

   xviii, 
  4, 
  310, 
  1910. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Yol. 
  XXXVI, 
  IS 
  o. 
  214. 
  — 
  October, 
  1913. 
  

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