﻿332 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  that 
  time 
  the 
  relations 
  were 
  supposed 
  to 
  be 
  much 
  more 
  simple 
  

   than 
  was 
  found 
  shortly 
  afterward 
  to 
  be 
  the 
  case. 
  This 
  was 
  

   due 
  principally 
  to 
  the 
  fact 
  that 
  the 
  mineral 
  cristobalite 
  was 
  

   almost 
  unknown 
  at 
  the 
  time, 
  and 
  only 
  the 
  relations 
  between 
  

   the 
  two 
  forms, 
  quartz 
  and 
  tridymite, 
  were 
  considered. 
  In 
  

   reality, 
  the 
  products 
  classed 
  as 
  tridymite 
  consisted 
  at 
  times 
  of 
  

   tridymite 
  and 
  at 
  other 
  times 
  of 
  cristobalite. 
  The 
  optical 
  

   properties 
  of 
  the 
  two 
  are 
  so 
  similar 
  that 
  the 
  fact 
  that 
  two 
  

   different 
  products 
  were 
  obtained 
  was 
  not 
  realized, 
  although 
  it 
  

   was 
  noted 
  that 
  the 
  index 
  of 
  refraction 
  in 
  some 
  preparations 
  

   was 
  slightly 
  higher 
  than 
  normal. 
  It 
  may 
  be 
  noted 
  also 
  that 
  

   the 
  values 
  of 
  refringence 
  and 
  birefringence 
  of 
  cristobalite 
  

   given 
  in 
  some 
  of 
  the 
  standard 
  mineralogies 
  are 
  in 
  error. 
  By 
  

   consulting 
  Mallard's* 
  original 
  paper, 
  from 
  which 
  they 
  are 
  

   quoted, 
  it 
  is 
  obvious 
  that 
  the 
  value 
  of 
  the 
  index 
  has 
  been 
  mis- 
  

   printed 
  and 
  that 
  of 
  the 
  birefringence 
  probably 
  misinterpreted. 
  

  

  In 
  a 
  second 
  paper 
  from 
  the 
  Geophysical 
  Laboratory,! 
  deal- 
  

   ing 
  with 
  the 
  matter, 
  attention 
  was 
  called 
  to 
  the 
  inaccuracy 
  in 
  

   some 
  of 
  the 
  statements 
  of 
  the 
  preceding 
  paper, 
  and 
  it 
  was 
  

   stated 
  : 
  " 
  Recent 
  work 
  on 
  the 
  silica 
  problem, 
  at 
  low 
  tempera- 
  

   tures, 
  has 
  shown 
  it 
  to 
  be 
  much 
  more 
  complicated 
  than 
  was 
  at 
  

   first 
  supposed. 
  In 
  fact, 
  several 
  phases 
  have 
  now 
  been 
  found 
  

   to 
  occur 
  in 
  that 
  region 
  which 
  were 
  not 
  disclosed 
  by 
  the 
  first 
  

   investigation. 
  The 
  problem 
  as 
  a 
  whole 
  is 
  not 
  simple, 
  and 
  has 
  

   not 
  yet 
  been 
  satisfactorily 
  solved, 
  so 
  that 
  in 
  the 
  following 
  

   paragraphs 
  only 
  a 
  report 
  of 
  progress 
  can 
  be 
  made." 
  

  

  Through 
  the 
  courtesy 
  of 
  Professor 
  Lacroix 
  of 
  Paris, 
  to 
  

   whom 
  specimens 
  of 
  the 
  artificial 
  crystals 
  obtained 
  by 
  the 
  devitri- 
  

   fication 
  of 
  silica 
  glass 
  and 
  by 
  heating 
  quartz 
  at 
  high 
  tempera- 
  

   tures, 
  had 
  been 
  sent, 
  it 
  had 
  been 
  shown 
  that 
  these 
  were 
  

   probably 
  cristobalite 
  and 
  not 
  tridymite 
  as 
  had 
  formerly 
  been 
  

   supposed. 
  The 
  acceptance 
  of 
  this 
  view, 
  however, 
  seemed 
  to 
  

   open 
  up 
  again 
  the 
  whole 
  question, 
  for 
  if 
  this 
  mineral 
  was 
  cristo- 
  

   balite, 
  what 
  was 
  the 
  position 
  of 
  tridymite 
  in 
  the 
  series 
  ? 
  In 
  

   fact, 
  beyond 
  the 
  determination 
  that 
  tridymite 
  and 
  cristobalite 
  

   were 
  high 
  temperature 
  forms, 
  nothing 
  seemed 
  certain 
  regard- 
  

   ing 
  their 
  relations. 
  This 
  uncertainty 
  was 
  increased 
  by 
  the 
  fact 
  

   that 
  other 
  investigators 
  had 
  reported 
  the 
  artificial 
  production 
  

   of 
  tridymite 
  and 
  cristobalite 
  under 
  such 
  conditions 
  that 
  it 
  

   seemed 
  even 
  a 
  question 
  whether 
  they 
  were 
  properly 
  high 
  tem- 
  

   perature 
  minerals. 
  The 
  manner 
  of 
  their 
  occurrence 
  in 
  nature 
  

   also 
  suggested 
  the 
  possibility 
  that 
  their 
  field 
  of 
  stability 
  was 
  in 
  

   the 
  region 
  below 
  800°. 
  On 
  the 
  other 
  hand, 
  Professor 
  Koenigs- 
  

  

  *E. 
  Mallard, 
  Bull. 
  Soc. 
  Min., 
  xiii, 
  175, 
  1890. 
  

  

  f 
  The 
  Binary 
  Systems 
  of 
  Alumina 
  with 
  Silica, 
  Lime 
  and 
  Magnesia, 
  E. 
  S. 
  

   Shepherd, 
  G. 
  A. 
  Rankin 
  and 
  F. 
  E. 
  Wright, 
  this 
  Journal, 
  (4), 
  xxviii, 
  293-833, 
  

   1909. 
  

  

  