﻿320 
  Posnjah 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  Goethite: 
  16 
  — 
  Fe 
  2 
  3 
  -H 
  2 
  0; 
  orthorhombic 
  with 
  a:b:c 
  = 
  

   about 
  0-90-0-92: 
  1: 
  0-600-0-605 
  ; 
  17 
  density 
  4-28 
  ± 
  0-01 
  ; 
  

   prismatic, 
  bladed, 
  fibrous; 
  18 
  elongation 
  parallel 
  to 
  c; 
  

   cleavage 
  perfect 
  b, 
  good 
  a; 
  refractive 
  indices 
  19 
  are 
  a 
  = 
  

   2-26, 
  p 
  = 
  2-394, 
  y 
  == 
  2-400 
  ; 
  fine 
  powder 
  is 
  dull 
  orange-yel- 
  

   low 
  (16" 
  i 
  to 
  16' 
  i 
  Ridgway) 
  ; 
  thicker 
  grains 
  show 
  slight 
  

   pleochroism 
  in 
  orange-yellow 
  browns 
  (16 
  I), 
  thinner 
  

   grains 
  are 
  not 
  perceptibly 
  pleochroic 
  and 
  are 
  clear 
  yel- 
  

   low 
  ; 
  for 
  red 
  a 
  = 
  b,$ 
  = 
  a, 
  y 
  = 
  c, 
  and 
  for 
  blue 
  a 
  = 
  b, 
  (3 
  = 
  c, 
  

   y==a; 
  P 
  = 
  y 
  at 
  610-620^. 
  Grains 
  lying 
  on 
  the 
  best 
  

   cleavage 
  face 
  show 
  a 
  very 
  characteristic 
  green 
  interfer- 
  

   ence 
  color, 
  while 
  colors 
  of 
  grains 
  slightly 
  tilted 
  from 
  this 
  

   position 
  pass 
  through 
  red 
  and 
  green 
  without 
  extinguish- 
  

   ing. 
  In 
  white 
  light 
  the 
  absorption 
  formula 
  is 
  a>c>b. 
  

  

  The 
  foregoing 
  description 
  applies 
  to 
  well-crystallized 
  

   goethite, 
  the 
  following 
  to 
  fibrous 
  goethite. 
  

  

  Aggregates 
  of 
  thin 
  blades 
  and 
  fibers, 
  subparallel 
  with 
  

   respect 
  to 
  the 
  axis 
  c, 
  vary 
  considerably 
  physically 
  and 
  

   chemically. 
  They 
  usually 
  appear 
  microscopically 
  homo- 
  

   geneous 
  but 
  their 
  refractive 
  indices 
  are 
  low 
  ; 
  they 
  contain 
  

   water, 
  silica, 
  etc., 
  as 
  impurities, 
  and 
  they 
  show 
  confused 
  

   interference 
  figures, 
  or 
  are 
  uniaxial 
  at 
  shorter 
  wave- 
  

   lengths 
  than 
  is 
  goethite 
  owing 
  to 
  lack 
  of 
  parallel 
  orienta- 
  

   tion 
  with 
  respect 
  to 
  a 
  and 
  b. 
  The 
  optical 
  properties 
  of 
  

   these 
  aggregates 
  are 
  best 
  explained 
  by 
  the 
  following 
  con- 
  

   siderations. 
  

  

  If 
  very 
  fine 
  (diameter 
  0-5/* 
  or 
  less) 
  fibers 
  or 
  blades 
  of 
  

   goethite 
  — 
  elongation 
  c 
  — 
  could 
  be 
  packed 
  without 
  pore 
  

   space 
  and 
  with 
  random 
  orientation 
  with 
  respect 
  to 
  a 
  and 
  

   b, 
  then 
  slivers 
  of 
  the 
  apparently 
  parallel-fibrous 
  aggre- 
  

   gates 
  would 
  have 
  a 
  maximum 
  refractive 
  index 
  parallel 
  to 
  

   the 
  elongation 
  equal 
  to 
  2-40, 
  and 
  a 
  minimum 
  20 
  throughout 
  

   the 
  plane 
  perpendicular 
  to 
  the 
  elongation 
  equal 
  to 
  2-33. 
  

   Absorption 
  would 
  be 
  slightly 
  greater 
  in 
  the 
  direction 
  of 
  

   the 
  elongation. 
  21 
  

  

  18 
  More 
  complete 
  descriptions 
  appear 
  near 
  the 
  end 
  of 
  the 
  paper. 
  

  

  17 
  Crystallographic 
  irregularities 
  limit 
  the 
  accuracy. 
  See 
  p. 
  342, 
  also 
  G. 
  

   Cesaro 
  and 
  A. 
  Abraham, 
  Bull. 
  Acad. 
  E. 
  Belg., 
  1903, 
  178. 
  . 
  

  

  18 
  See 
  next 
  section. 
  

  

  10 
  Dr. 
  E. 
  S. 
  Larsen, 
  of 
  the 
  U. 
  S. 
  Geol. 
  Survey, 
  has 
  found 
  a 
  Li 
  = 
  2-21, 
  p 
  u 
  

   and=7u 
  2-33-2-35 
  (unpublished). 
  

  

  20 
  That 
  is 
  the 
  maximum 
  would 
  equal 
  y 
  of 
  goethite 
  and 
  the 
  minimum 
  " 
  

  

  approximately, 
  and 
  the 
  aggregates 
  would 
  be 
  essentially 
  uniaxial 
  positive 
  

   with 
  u 
  = 
  2-33 
  and 
  e 
  = 
  2-40. 
  

  

  21 
  The 
  diffusion 
  of 
  light 
  at 
  boundaries 
  between 
  differently 
  refracting 
  

   surfaces 
  may 
  affect 
  the 
  apparent 
  pleochoism 
  of 
  fibrous 
  substances. 
  Grains 
  

   should 
  be 
  observed 
  in 
  a 
  medium 
  having 
  a 
  refractive 
  index 
  midway 
  between 
  

   the 
  refractive 
  indices 
  of 
  the 
  differently 
  absorbed 
  rays. 
  

  

  