﻿354 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  sider 
  entirely 
  satisfactory. 
  The 
  mean 
  index, 
  he 
  says, 
  is 
  near 
  

   1*49. 
  For 
  artificial 
  crystals 
  prepared 
  in 
  a 
  tungstate 
  melt 
  I 
  

   have 
  determined 
  the 
  indices 
  in 
  the 
  manner 
  described 
  for 
  

   quartz, 
  and 
  obtained 
  7 
  = 
  1/487 
  a 
  = 
  1-484 
  (sodium 
  light, 
  tem- 
  

   perature 
  24°). 
  Mallard 
  determined 
  the 
  value 
  of 
  the 
  bire- 
  

   fringence 
  as 
  0'00053. 
  There 
  seems 
  to 
  be 
  here 
  also 
  an 
  error 
  of 
  

   some 
  kind. 
  At 
  any 
  rate, 
  the 
  artificial 
  crystals 
  show 
  a 
  bire- 
  

   fringence 
  nearly 
  equal 
  to 
  that 
  of 
  tridymite, 
  probably 
  a 
  little 
  

   less. 
  M. 
  Bauer, 
  describing 
  the 
  crystals 
  of 
  vom 
  Rath's* 
  original 
  

   discovery, 
  speaks 
  of 
  the 
  " 
  ziemlich 
  kraf 
  tige 
  Doppelbrechung." 
  

  

  Determination 
  of 
  the 
  specific 
  gravity 
  by 
  the 
  method 
  of 
  Day 
  

   and 
  Allen 
  gave 
  2*333 
  for 
  cristobalite 
  at 
  27° 
  referred 
  to 
  water 
  

   at 
  27°. 
  Mallard 
  found 
  2*34 
  for 
  natural 
  crystals. 
  

  

  As 
  ordinarily 
  obtained, 
  the 
  artificial 
  cristobalite 
  shows 
  con- 
  

   siderable 
  general 
  resemblance 
  to 
  the 
  elongated 
  form 
  of 
  tridy- 
  

   mite. 
  The 
  difference 
  in 
  indices 
  of 
  refraction, 
  however, 
  

   while 
  slight, 
  is 
  sufficient 
  ordinarily 
  for 
  discrimination. 
  More- 
  

   over, 
  the 
  extinction 
  of 
  cristobalite 
  in 
  such 
  aggregates 
  is 
  not 
  

   parallel 
  to 
  any 
  recognizable 
  crystallographic 
  feature, 
  and 
  

   again, 
  cristobalite 
  grains 
  frequently 
  show 
  a 
  distinct 
  polysyn- 
  

   thetic 
  twinning 
  like 
  that 
  of 
  albite, 
  or 
  a 
  plaid 
  effect 
  like 
  micro- 
  

   cline. 
  It 
  is 
  a 
  fact 
  not 
  without 
  significance 
  in 
  considering 
  the 
  

   possibility 
  of 
  material 
  being 
  taken 
  up 
  in 
  solution 
  by 
  cristoba- 
  

   lite 
  formed 
  in 
  tungstate 
  melts, 
  that 
  cristobalite, 
  unlike 
  quartz 
  

   and 
  tridymite, 
  can 
  be 
  formed 
  without 
  a 
  fiux 
  and 
  the 
  material. 
  

   so 
  prepared 
  does 
  not 
  differ 
  observably 
  from 
  that 
  formed 
  with 
  

   a 
  flux. 
  Even 
  those 
  preparations 
  obtained 
  by 
  aid 
  of 
  a 
  flux 
  

   show 
  very 
  little 
  impurity, 
  0'19-0'35 
  per 
  cent 
  according 
  to 
  

   several 
  analyses. 
  

  

  The 
  best 
  crystals 
  of 
  cristobalite 
  have 
  been 
  obtained 
  by 
  heat- 
  

   ing 
  amorphous 
  silica 
  with 
  sodic 
  tungstate 
  over 
  a 
  Bunsen 
  

   burner. 
  They 
  then 
  show 
  an 
  attempt 
  to 
  develop 
  a 
  definite 
  

   crystal 
  form, 
  but 
  generally 
  arrive 
  at 
  no 
  better 
  results 
  than 
  the 
  

   forms 
  illustrated 
  in 
  fig. 
  4. 
  Many 
  of 
  the 
  dihedral 
  and 
  poly- 
  

   hedral 
  angles 
  are 
  nearly 
  perfect, 
  but 
  the 
  remainder 
  of 
  the 
  

   crystal 
  is 
  a 
  mere 
  skeleton 
  framework. 
  At 
  times 
  the 
  principal 
  

   growth 
  has 
  been 
  in 
  the 
  direction 
  of 
  one 
  axis 
  only, 
  more 
  often 
  

   along 
  two 
  or 
  three 
  at 
  right 
  angles 
  to 
  each 
  other. 
  Crystals 
  fre- 
  

   quently 
  show 
  many 
  more 
  branches 
  than 
  those 
  illustrated, 
  but 
  

   the 
  general 
  form 
  of 
  growth 
  has 
  been 
  the 
  same. 
  

  

  In 
  every 
  case 
  where 
  terminal 
  caps 
  have 
  been 
  developed 
  they 
  

   appear 
  to 
  be 
  octahedra. 
  It 
  is 
  not 
  possible 
  to 
  get 
  very 
  exact 
  

   measurements 
  of 
  such 
  small 
  crystals 
  under 
  the 
  microscope, 
  

   but, 
  as 
  nearly 
  as 
  determinable, 
  the 
  edges 
  make 
  angles 
  of 
  90° 
  

   with 
  each 
  other 
  and 
  the 
  plane 
  faces 
  make 
  angles 
  of 
  practically 
  

   70°. 
  From 
  the 
  relations 
  of 
  the 
  caps 
  to 
  the 
  axes 
  of 
  elongation, 
  

  

  *G. 
  vom 
  Rath 
  and 
  Max 
  Bauer, 
  Neues 
  Jahrb., 
  i, 
  200, 
  1887. 
  

  

  