﻿380 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  very 
  difficult 
  of 
  interpretation 
  and 
  can 
  hardly 
  be 
  considered 
  as 
  

   evidence. 
  Frequently 
  the 
  specimens 
  of 
  chalcedony 
  were 
  given 
  a 
  

   preliminary 
  heating 
  and 
  only 
  those 
  which 
  did 
  not 
  become 
  

   badly 
  cracked 
  were 
  used 
  in 
  the 
  experimental 
  work. 
  The 
  

   degree 
  of 
  heating 
  is 
  important 
  but 
  is 
  seldom 
  mentioned. 
  In 
  

   one 
  case,* 
  however, 
  rods 
  of 
  chalcedony 
  whose 
  expansion 
  was 
  

   to 
  be 
  determined 
  were 
  placed 
  in 
  a 
  porcelain-furnace 
  at 
  1500°. 
  

   This 
  naturally 
  converted 
  the 
  chalcedony 
  into 
  cristobalite, 
  and 
  

   the 
  sudden 
  increase 
  in 
  expansion 
  between 
  170° 
  and 
  245° 
  which 
  

   he 
  found 
  is 
  what 
  would 
  be 
  expected. 
  Le 
  Chatelier 
  speaks 
  of 
  

   the 
  uncertainty^ 
  caused 
  by 
  the 
  multitude 
  of 
  fissures 
  which 
  

   traverse 
  the 
  specimens 
  after 
  heating 
  and 
  affect 
  the 
  results. 
  

   I 
  have 
  replotted 
  the 
  values 
  given 
  for 
  a 
  number 
  of 
  experiments 
  

   and 
  am 
  not 
  able 
  to 
  perceive 
  any 
  definite 
  evidence 
  of 
  sudden 
  

   expansion 
  corresponding 
  to 
  that 
  of 
  quartz 
  at 
  575°. 
  

  

  As 
  quartz 
  gives 
  a 
  plainly-marked 
  break 
  on 
  the 
  heating 
  and 
  

   cooling 
  curves 
  when 
  passing 
  through 
  the 
  inversion-point, 
  chal- 
  

   cedony 
  should 
  give 
  similar 
  breaks 
  if 
  it 
  is 
  identical 
  with 
  quartz. 
  

   To 
  determine 
  this 
  a 
  number 
  of 
  samples 
  of 
  chalcedony 
  were 
  

   tested 
  in 
  the 
  manner 
  described 
  under 
  quartz. 
  The 
  specimens 
  

   were 
  from 
  various 
  localities. 
  One 
  was 
  from 
  the 
  state 
  of 
  

   Chihuahua, 
  Mexico, 
  another 
  from 
  Tampa 
  Bay, 
  Florida, 
  and 
  a 
  

   third 
  from 
  Kerguelen 
  Island. 
  The 
  appearance 
  of 
  all 
  was 
  

   that 
  of 
  typical 
  chalcedony. 
  They 
  were 
  nearly 
  transparent, 
  

   waxy, 
  and 
  of 
  a 
  botryoidal 
  form, 
  and 
  under 
  the 
  microscope 
  

   showed 
  a 
  finely 
  fibrous 
  development. 
  The 
  details 
  of 
  experi- 
  

   mentation 
  were 
  made 
  precisely 
  the 
  same 
  as 
  for 
  quartz. 
  

   Heating 
  and 
  cooling 
  curves 
  were 
  run 
  and 
  the 
  region 
  on 
  both 
  

   sides 
  of 
  575° 
  was 
  carefully 
  explored, 
  but 
  no 
  indication 
  of 
  a 
  

   break 
  in 
  the 
  curves 
  was 
  perceptible. 
  

  

  In 
  another 
  form 
  of 
  investigation 
  powdered 
  chalcedony 
  from 
  

   the 
  localities 
  mentioned 
  was 
  mixed 
  with 
  sodic 
  tungstate 
  and 
  

   heated 
  at 
  a 
  temperature 
  of 
  750° 
  to 
  850°. 
  It 
  will 
  be 
  remembered 
  

   that 
  amorphous 
  silica, 
  when 
  so 
  treated, 
  gave 
  tridymite 
  at 
  first 
  

   and 
  quartz 
  later, 
  and 
  the 
  reason 
  for 
  this 
  has 
  been 
  discussed. 
  

   For 
  the 
  same 
  reason 
  chalcedony, 
  if 
  unstable 
  at 
  these 
  tempera- 
  

   tures, 
  might 
  be 
  expected 
  to 
  act 
  similarly, 
  but 
  if 
  it 
  is 
  the 
  same 
  

   mineral 
  as 
  quartz 
  no 
  change 
  could 
  be 
  expected 
  to 
  occur 
  except 
  

   that 
  quartz 
  crystals 
  might 
  conceivably 
  grow 
  to 
  a 
  larger 
  size 
  in 
  

   the 
  melt. 
  In 
  all 
  cases 
  the 
  result 
  was 
  that 
  new 
  quartz 
  crystals 
  

   were 
  formed 
  and 
  usually 
  tridymite 
  also, 
  often 
  in 
  great 
  

   abundance. 
  If 
  only 
  a 
  small 
  amount 
  of 
  tridymite 
  were 
  formed 
  

   under 
  such 
  conditions 
  its 
  origin 
  might 
  be 
  attributed 
  to 
  inter- 
  

   mixed 
  amorphous 
  (opaline) 
  silica 
  in 
  the 
  original 
  chalcedony, 
  

   but 
  the 
  quantity 
  is 
  so 
  large 
  that 
  this 
  hardly 
  seems 
  possible. 
  

  

  *Le 
  Chatelier, 
  Compt. 
  rend., 
  cxi, 
  123, 
  1890. 
  

   fLe 
  Chatelier, 
  ibid., 
  cviii, 
  1046, 
  1889. 
  

  

  