﻿356 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  The 
  general 
  crystallographic 
  symmetry 
  of 
  cristobalite 
  indi- 
  

   cates 
  that 
  under 
  the 
  conditions 
  of 
  formation 
  it 
  is 
  actually 
  an 
  

   isometric 
  mineral, 
  but 
  in 
  cooling 
  to 
  ordinary 
  temperatures 
  it 
  

   passes 
  through 
  an 
  inversion, 
  by 
  which 
  it 
  becomes 
  birefringent. 
  

   In 
  the 
  sketches 
  of 
  cristobalite 
  crystals 
  shown 
  in 
  fig. 
  4, 
  dotted 
  

   lines 
  indicate 
  the 
  birefringent 
  fields 
  and 
  arrows 
  show 
  vibra- 
  

   tion 
  directions. 
  Crossed 
  arrows 
  in 
  a 
  circle 
  show 
  that 
  no 
  bire- 
  

   fringence 
  is 
  perceptible. 
  In 
  the 
  last 
  case, 
  the 
  crystals 
  should 
  

   be 
  perpendicular 
  to 
  an 
  optic 
  axis, 
  or 
  nearly 
  so, 
  but 
  because 
  of 
  

   the 
  weak 
  birefringence 
  of 
  the 
  mineral 
  and 
  the 
  small 
  thickness 
  

   of 
  the 
  crystal, 
  no 
  indication 
  whatever 
  of 
  an 
  interference 
  figure 
  

   could 
  be 
  perceived 
  in 
  convergent 
  light. 
  The 
  sections 
  which 
  

   showed 
  maximum 
  birefringence, 
  however, 
  gave 
  a 
  figure 
  appar- 
  

   ently 
  perpendicular 
  to 
  an 
  optic 
  normal 
  (/3). 
  From 
  this 
  it 
  was 
  

   possible 
  to 
  determine 
  that 
  the 
  acute 
  bisectrix 
  is 
  a 
  and 
  hence 
  

   the 
  mineral 
  is 
  negative. 
  This 
  agrees 
  with 
  Mallard's 
  determi- 
  

   nation 
  on 
  natural 
  crystals. 
  The 
  manner 
  in 
  which 
  the 
  bire- 
  

   fringent 
  fields 
  are 
  arranged 
  also 
  agrees 
  with 
  Mallard's 
  

   observations. 
  The 
  plane 
  of 
  secondary 
  twinning 
  is 
  generally 
  

   quite 
  sharp 
  and 
  makes 
  an 
  angle 
  of 
  45° 
  or 
  90° 
  with 
  the 
  cubic 
  

   axis 
  which 
  it 
  crosses. 
  In 
  some 
  instances, 
  however, 
  the 
  border 
  

   of 
  adjacent 
  fields 
  is 
  quite 
  irregular, 
  as 
  in 
  f. 
  The 
  position 
  of 
  

   the 
  secondary 
  twinning 
  plane 
  and 
  the 
  relations 
  which 
  the 
  

   vibration 
  directions 
  bear 
  to 
  each 
  other 
  is 
  concordant 
  with 
  the 
  

   idea 
  of 
  tetragonal 
  or 
  orthorhombic* 
  symmetry 
  of 
  the 
  low- 
  tem- 
  

   perature 
  form, 
  with 
  twinning 
  after 
  a 
  45° 
  pyramid 
  parallel 
  to 
  

   an 
  octahedral 
  edge 
  of 
  the 
  original 
  crystal. 
  During 
  inversion, 
  

   therefore, 
  the 
  crystallographic 
  space-lattice 
  seems 
  to 
  suffer 
  but 
  

   little 
  distortion. 
  The 
  tendency 
  to 
  assume 
  skeleton 
  forms 
  

   agrees 
  with 
  the 
  description 
  of 
  natural 
  crystals, 
  as 
  does 
  the 
  

   occurrence 
  of 
  twinning 
  after 
  the 
  spinel 
  law.f 
  

  

  Preparation 
  of 
  Quartz 
  in 
  Aqueous 
  Solution. 
  

  

  Quartz 
  may 
  be 
  prepared 
  without 
  difficulty 
  by 
  heating 
  either 
  

   silica 
  glass 
  or 
  amorphous 
  precipitated 
  silica 
  with 
  water 
  and 
  

   sodic 
  carbonate 
  in 
  a 
  silver-lined 
  steel 
  bomb 
  at 
  400° 
  to 
  500° 
  for 
  

   two 
  or 
  three 
  days. 
  The 
  relative 
  proportions 
  of 
  materials 
  need 
  

   not 
  be 
  very 
  exact 
  ; 
  approximately 
  the 
  following 
  were 
  used 
  in 
  

   several 
  experiments 
  : 
  water 
  8 
  CC 
  , 
  silica 
  2-3 
  g., 
  crystallized 
  sodic 
  

   carbonate 
  0*7 
  g., 
  capacity 
  of 
  bomb 
  16 
  cc 
  . 
  

  

  Experiments 
  of 
  this 
  kind 
  have 
  been 
  performed 
  a 
  number 
  of 
  

   times 
  and 
  have 
  no 
  special 
  interest. 
  Of 
  more 
  importance 
  was 
  

   an 
  investigation 
  as 
  to 
  the 
  possibility 
  of 
  obtaining 
  tridymite 
  or 
  

  

  *E. 
  Mallard, 
  Bull. 
  Soc. 
  Min., 
  xiii, 
  175, 
  1890. 
  A. 
  Lacroix, 
  ibid., 
  xiv, 
  186, 
  

   1891. 
  

  

  t 
  G. 
  vom 
  Bath, 
  Neues 
  Jahrb., 
  i, 
  198, 
  1887. 
  P. 
  Gaubert, 
  Bull. 
  Soc. 
  Min., 
  

   xxvii, 
  242, 
  1904. 
  

  

  