﻿352 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  1469 
  (difference 
  too 
  small 
  to 
  be 
  determined), 
  perpendicular 
  to 
  

   plates 
  (7) 
  1-4:73 
  (temp. 
  24°). 
  All 
  these 
  characteristics 
  agree 
  

   closely 
  with 
  those 
  of 
  the 
  natural 
  mineral, 
  as 
  given 
  by 
  Mallard,* 
  

   whose 
  determinations 
  and 
  descriptions 
  are 
  usually 
  cited. 
  Mal- 
  

   lard 
  found 
  difficulties 
  in 
  exact 
  determinations 
  of 
  optical 
  con- 
  

   stants, 
  but 
  gives 
  -^ 
  JL 
  = 
  1477, 
  7 
  - 
  a 
  = 
  0*00185, 
  2 
  

  

  E 
  = 
  

  

  66° 
  about, 
  and 
  2V 
  = 
  43° 
  about. 
  

  

  A 
  determination 
  of 
  specific 
  gravity 
  was 
  made 
  by 
  the 
  method 
  

   of 
  Day 
  and 
  Allen. 
  f 
  The 
  value 
  found 
  was 
  2*270 
  for 
  tridymite 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  3. 
  Tridymite 
  crystal 
  in 
  basal 
  position 
  ; 
  length 
  about 
  1'0 
  E 
  

  

  at 
  27° 
  referred 
  to 
  water 
  at 
  27°. 
  Mallard 
  gives 
  2*28 
  for 
  the 
  

   natural 
  mineral. 
  

  

  The 
  optical 
  characteristics 
  of 
  the 
  low 
  temperature 
  (a 
  — 
  ) 
  

   form 
  of 
  tridymite 
  indicate 
  orthorhombic 
  symmetry. 
  Each 
  

   hexagonal 
  plate 
  appears 
  to 
  be 
  made 
  up 
  of 
  several 
  orthorhom- 
  

   bic 
  individuals 
  whose 
  vertical 
  axes 
  are 
  parallel 
  with 
  the 
  vertical 
  

   axis 
  of 
  the 
  hexagonal 
  crystals, 
  but 
  which 
  are 
  twinned 
  after 
  

   a 
  60° 
  orthorhombic 
  prism 
  coinciding 
  with 
  the 
  60° 
  hexagonal 
  

  

  *E. 
  Mallard, 
  Bull. 
  Soc. 
  Min., 
  xiii, 
  161, 
  1890. 
  

  

  f 
  Publication 
  No. 
  31, 
  Carnegie 
  Inst, 
  of 
  Washington, 
  p. 
  55, 
  1905. 
  

  

  