﻿342 
  Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  rically. 
  One, 
  from 
  excellent 
  faces 
  e, 
  d, 
  m, 
  gave 
  the 
  ratios 
  

   a:b 
  :c 
  = 
  0-928 
  : 
  1 
  : 
  0-604 
  ; 
  the 
  second, 
  although 
  giving 
  good 
  

   signal 
  from 
  faces 
  p, 
  e, 
  d, 
  m, 
  b, 
  gave 
  less 
  satisfactory 
  axial 
  

   ratios 
  (a:bw 
  = 
  0-931: 
  1: 
  0-605) 
  because 
  two 
  of 
  the 
  pyra- 
  

   mid 
  faces 
  were 
  rotated 
  toward 
  a 
  40' 
  from 
  their 
  true 
  posi- 
  

   tion 
  over 
  m. 
  Near 
  the 
  base 
  of 
  each 
  crystal 
  was 
  a 
  compact 
  

   group 
  .of 
  small 
  crystals 
  in 
  sub-parallel 
  positions. 
  Uni- 
  

   axial 
  at 
  610 
  to 
  615/*/*, 
  at 
  longer 
  wave-lengths 
  the 
  optic 
  

   plane 
  is 
  parallel 
  to 
  the 
  elongation. 
  a(Na) 
  = 
  2-260 
  ± 
  

   0-005, 
  /?(Na) 
  = 
  2-393 
  ± 
  0-005, 
  y 
  (Na) 
  = 
  2-398 
  ± 
  0-005. 
  

   Absorption 
  increases 
  abruptly 
  near 
  550/*/* 
  as 
  short 
  wave- 
  

   lengths 
  are 
  approached. 
  

  

  Goethite, 
  Cornwall, 
  England. 
  54 
  — 
  Coarse 
  blades 
  which 
  

   have 
  /?(Na)=2-39 
  4 
  and 
  y(Na)=2-40 
  . 
  Uniaxial 
  near 
  

   610/*/*. 
  Dispersion 
  of 
  y(Na) 
  — 
  y(Li)=0-60. 
  

  

  Goethite, 
  Florissant, 
  Colorado. 
  54 
  (N. 
  M. 
  84940). 
  Uni- 
  

   axial 
  at 
  616 
  to 
  620/*/*. 
  

  

  Goethite, 
  Pribram, 
  Bohemia, 
  variety 
  Sammetblende. 
  — 
  

   Slender 
  separate 
  filaments 
  like 
  the 
  nap 
  of 
  velvet. 
  The 
  

   filaments, 
  although 
  only 
  a 
  few 
  /* 
  in 
  diameter, 
  are 
  clearly 
  

   made 
  up 
  of 
  small, 
  nearly 
  parallel, 
  fibers 
  or 
  blades 
  elon- 
  

   gated 
  ± 
  a. 
  Some 
  filaments 
  show 
  the 
  interference 
  colors 
  

   characteristic 
  of 
  goethite 
  but 
  interference 
  figures 
  (which 
  

   were 
  not 
  very 
  satisfactory) 
  indicated 
  the 
  optic 
  plane 
  

   parallel 
  to 
  the 
  elongation, 
  and 
  a 
  slowly 
  decreasing 
  optic 
  

   angle 
  (toward 
  blue). 
  At 
  600/*/* 
  2E 
  == 
  about 
  80°. 
  

  

  y(Na) 
  =2-36 
  , 
  £(Na) 
  =2-35 
  , 
  a(Na) 
  = 
  2-23 
  5 
  . 
  Disper- 
  

   sion 
  o(Na)— 
  a(Li)= 
  0-040. 
  

  

  Goethite, 
  labelled 
  limonite, 
  Diamond 
  Hill, 
  Rhode 
  

   Island. 
  55 
  (N. 
  M. 
  17879.) 
  The 
  specimen 
  consists 
  of 
  

   radial-fibrous 
  aggregates 
  which 
  break 
  radially 
  with 
  glis- 
  

   tening 
  surfaces. 
  A 
  bladed 
  to 
  fibrous 
  structure 
  is 
  revealed 
  

   by 
  the 
  microscope, 
  and 
  interference 
  colors 
  like 
  goethite 
  

   appear 
  in 
  some 
  grains. 
  These 
  grains 
  are 
  uniaxial 
  at 
  575 
  

   to 
  590/*/*, 
  and 
  have 
  2E 
  at 
  675/*/* 
  and 
  at 
  546/*/* 
  90° 
  to 
  120°, 
  or 
  

   2V==35° 
  to 
  40°. 
  For 
  these 
  grains 
  <* 
  = 
  2-24 
  and 
  both 
  

   p 
  and 
  y 
  = 
  2-37; 
  <* 
  is 
  less 
  absorbed. 
  Dispersion 
  y(Na) 
  — 
  

   y( 
  Li) 
  =0-053. 
  

  

  Goethite, 
  Brazil. 
  55 
  (N. 
  M. 
  80862.) 
  Botryoidal 
  and 
  

   stalactitic 
  masses 
  break 
  radially 
  with 
  semi-lustrous 
  sur- 
  

   faces. 
  Relatively 
  few 
  grains 
  in 
  a 
  crushed 
  sample 
  

   resemble 
  goethite. 
  These 
  are 
  nearly 
  uniaxial 
  at 
  575/*/*, 
  

   and 
  have 
  variable 
  refractive 
  indices 
  : 
  a, 
  about 
  2-23, 
  p 
  

  

  r,i 
  Analyzed, 
  see 
  Table 
  I. 
  

   Analyzed 
  ( 
  Table 
  I). 
  

  

  