﻿Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  343 
  

  

  about 
  2-32, 
  y 
  about 
  2-35. 
  Material 
  which 
  is 
  more 
  fibrous 
  

   has 
  a 
  about 
  2-21 
  and 
  /? 
  and 
  y 
  about 
  2-31. 
  

  

  Goethite 
  (labelled 
  xanthosiderite), 
  Neudorf, 
  Ger- 
  

   many. 
  55 
  (N. 
  M. 
  84270.) 
  Radial-fibrous, 
  decidedly 
  yel- 
  

   lowish, 
  interbanded 
  with 
  quartz. 
  Microscopic 
  fragments 
  

   very 
  cloudy 
  with 
  very 
  minute 
  inclusions 
  or 
  pores 
  — 
  which 
  

   accounts 
  for 
  yellow 
  color, 
  y 
  near 
  2-37, 
  (3 
  near 
  2-33, 
  a 
  near 
  

   2-27, 
  all 
  variable. 
  

  

  Goethite 
  (labelled 
  xanthosiderite), 
  Ironwood, 
  Mich- 
  

   igan. 
  56 
  (Holden 
  — 
  K.) 
  — 
  Long 
  fibers, 
  yellow, 
  translucent 
  

   in 
  microscopic 
  grains, 
  becomes 
  slowly 
  saturated 
  with 
  

   immersion 
  liquids 
  and 
  thus 
  becomes 
  transparent. 
  Its 
  

   yellow 
  color 
  in 
  mass 
  is 
  evidently 
  due 
  to 
  invisible 
  pores. 
  

   y 
  varies 
  around 
  2-33 
  and 
  a 
  around 
  2-25, 
  as 
  measured 
  on 
  

   material 
  saturated 
  with 
  these 
  immersion 
  liquids. 
  

  

  Fibrous 
  goethite. 
  — 
  A 
  number 
  of 
  specimens 
  marked 
  

   "limonite" 
  have 
  the 
  following 
  characteristics 
  in 
  common 
  : 
  

   in 
  the 
  mass 
  dark 
  yellow-brown 
  ; 
  breaking 
  in 
  radial 
  splin- 
  

   ters, 
  which 
  are 
  transparent 
  microscopically, 
  and 
  behave 
  

   much 
  like 
  an 
  optically 
  positive 
  mineral 
  with 
  prismatic 
  

   cleavage. 
  In 
  some 
  specimens 
  lack 
  of 
  strict 
  coincidence 
  

   of 
  the 
  prismatic 
  axis 
  of 
  small 
  groups 
  of 
  fibers 
  caused 
  

   slight 
  variation 
  in 
  the 
  measured 
  refractive 
  indices. 
  

   Other 
  larger 
  variations 
  such 
  as 
  would 
  result 
  from 
  struc- 
  

   tures, 
  described 
  on 
  p. 
  320, 
  or 
  from 
  lack 
  of 
  chemical 
  homo- 
  

   geneity 
  amounted 
  usually 
  to 
  less 
  than 
  ± 
  -02. 
  Table 
  IX 
  

   shows 
  other 
  optical 
  properties 
  of 
  these 
  specimens. 
  The 
  

   values 
  given 
  represent 
  as 
  nearly 
  as 
  could 
  be 
  determined 
  

   the 
  bulk 
  of 
  the 
  material, 
  taking 
  account 
  of 
  observed 
  vari- 
  

   ations. 
  Absorption 
  of 
  light 
  in 
  the 
  two 
  principal 
  direc- 
  

   tions 
  was 
  almost 
  identical 
  when 
  determined 
  in 
  a 
  liquid 
  of 
  

   intermediate 
  refractive 
  index 
  ; 
  perceptible 
  differences 
  

   showed 
  y 
  > 
  a. 
  

  

  Table 
  IX. 
  

   Designation 
  

  

  *Chatfield, 
  Conn., 
  N.M. 
  17302 
  

   N.M. 
  13972 
  a 
  

   •Polevskoi, 
  Urals. 
  N.M. 
  40352 
  

   *N.M. 
  Specimen 
  A. 
  

   *Moselem, 
  Pa., 
  from 
  E. 
  T. 
  Wherry 
  

   •N.M. 
  46746 
  

   N.M. 
  14116 
  

   N.M. 
  83867 
  

   From 
  a 
  geode, 
  E. 
  T. 
  Wherry 
  

  

  * 
  Analyzed, 
  see 
  Table 
  I. 
  

   59 
  The 
  sample 
  analyzed 
  (Table 
  I) 
  labelled 
  " 
  goethite 
  Mich." 
  was 
  similar 
  

   optically. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XL 
  VII, 
  No. 
  281.— 
  May, 
  1919. 
  

   24 
  

  

  7 
  

  

  a 
  Eemarks 
  

  

  2-26 
  

  

  2-18 
  Very 
  homogeneous 
  

  

  2-24 
  

  

  2- 
  16 
  Variable 
  indices 
  

  

  2-24 
  

  

  216 
  

  

  2-23 
  

  

  214 
  

  

  2-22 
  

  

  214 
  j 
  y 
  distinctly 
  more 
  

  

  217 
  

  

  2- 
  09 
  I 
  absorbed 
  

  

  2-23 
  

  

  214 
  

  

  2-21 
  

  

  213 
  

  

  2-20 
  

  

  213 
  

  

  