﻿Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  323 
  

  

  scope, 
  were 
  made 
  by 
  Cesaro 
  and 
  Abraham. 
  26 
  They 
  used 
  

   a 
  different 
  optical 
  orientation 
  from 
  that 
  of 
  either 
  Samoj- 
  

   loff 
  or 
  Lacroix 
  and 
  classed 
  their 
  crystals 
  as 
  a 
  special 
  type 
  

   of 
  goethite. 
  

  

  Our 
  determinations 
  of 
  refractive 
  index 
  and 
  density 
  

   have 
  confirmed 
  the 
  idea 
  that 
  these 
  crystals 
  are 
  entirely 
  

   distinct 
  from 
  goethite 
  ; 
  on 
  the 
  other 
  hand 
  their 
  composi- 
  

   tion 
  is 
  like 
  goethite. 
  

  

  By 
  adopting 
  the 
  name 
  and 
  optical 
  orientation 
  proposed 
  

   by 
  Lacroix, 
  and 
  by 
  combining 
  our 
  own 
  crystallographic 
  

   data 
  with 
  those 
  of 
  Cesaro 
  and 
  Abraham, 
  we 
  can 
  write 
  a 
  

   fairly 
  complete 
  synoptical 
  description 
  of 
  the 
  mineral. 
  

   For 
  more 
  details 
  see 
  fig. 
  1 
  and 
  the 
  description 
  of 
  speci- 
  

   mens, 
  p. 
  345. 
  

  

  Lepidocrocite, 
  Fe 
  2 
  3 
  -H 
  2 
  0, 
  orthorhombic, 
  a:b:c 
  = 
  0-4t3: 
  

   1 
  : 
  0-64. 
  Density 
  4-09 
  ± 
  -04. 
  Habit, 
  single 
  thin 
  scales 
  

   { 
  010 
  \ 
  , 
  slightly 
  elongated 
  c, 
  or 
  in 
  scaly 
  to 
  compact 
  bladed 
  

   aggregates. 
  Cleavage, 
  j 
  010 
  \ 
  perfect, 
  j 
  001 
  f 
  less 
  perfect, 
  

   100 
  \ 
  good. 
  Streak 
  dullish 
  orange. 
  Color 
  of 
  fine 
  pow- 
  

   der 
  13' 
  i 
  (Bidgway). 
  Refractive 
  indices: 
  a 
  = 
  1-94, 
  p 
  = 
  

   2-20, 
  y 
  = 
  2-51, 
  a 
  = 
  b,p 
  = 
  a, 
  y 
  = 
  c. 
  Therefore 
  — 
  2V==83°, 
  

   axial 
  dispersion 
  very 
  slight. 
  Pleochroism: 
  in 
  thicker 
  

   grains 
  a 
  is 
  clear 
  yellow, 
  p 
  dark 
  red-orange, 
  y 
  darker 
  

   orange-red; 
  in 
  thinner 
  grains 
  a 
  is 
  nearly 
  colorless 
  (yel- 
  

   lowish), 
  p 
  and 
  y 
  orange 
  to 
  yellow 
  according 
  to 
  thickness. 
  

  

  Turgite: 
  27 
  — 
  The 
  fibrous 
  red 
  oxide 
  of 
  iron 
  containing 
  

   variable 
  amounts 
  of 
  water 
  cannot 
  be 
  definitely 
  character- 
  

   ized. 
  Some 
  specimens 
  have 
  primary 
  structure, 
  others 
  

   are 
  pseudomorphic. 
  The 
  primary 
  structure 
  is 
  divergent- 
  

   fibrous. 
  Detached 
  splinters 
  have 
  parallel 
  extinction, 
  

   negative 
  elongation 
  and 
  negative 
  (apparently 
  uniaxial) 
  

   optical 
  character. 
  The 
  refractive 
  indices 
  thus 
  far 
  

   observed 
  28 
  range 
  between 
  a(ore)=2-3 
  to 
  2-6, 
  p 
  and 
  

   y 
  (or 
  a>) 
  =2-5 
  to 
  2-7. 
  The 
  color 
  in 
  fine 
  powder 
  is 
  distinctly 
  

   lighter 
  and 
  more 
  inclined 
  toward 
  orange 
  than 
  hematite. 
  

   The 
  corresponding 
  density 
  is 
  about 
  4-7, 
  and 
  the 
  water 
  

   content 
  about 
  4 
  to 
  6 
  per 
  cent. 
  

  

  Pseudomorphic 
  turgite 
  appears 
  to 
  result 
  from 
  the 
  par- 
  

   tial 
  dehydration 
  of 
  fibrous 
  goethite 
  and 
  primary 
  turgite, 
  

   and 
  from 
  the 
  oxidation 
  and 
  slight 
  hydration 
  of 
  magnetite 
  

  

  26 
  Bull. 
  Acad. 
  Belg., 
  1903, 
  178. 
  

  

  27 
  See 
  p. 
  347. 
  

  

  28 
  These 
  include 
  unpublished 
  values 
  kindly 
  furnished 
  by 
  Dr. 
  E. 
  S. 
  Larsen 
  

   of 
  the 
  TJ. 
  S. 
  Geological 
  Survey. 
  Mixtures 
  containing 
  sulphur, 
  iodine, 
  arsenic, 
  

   selenium 
  and 
  tellurium 
  are 
  suitable 
  for 
  making 
  such 
  measurements. 
  

  

  