﻿Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  347 
  

  

  tive, 
  extinction 
  parallel, 
  contains 
  many 
  thin 
  quartz 
  veins 
  ; 
  

   fibers 
  cloudy 
  and 
  only 
  translucent 
  to 
  feebly 
  transparent 
  ; 
  

   refractive 
  indices 
  variable, 
  but 
  the 
  lowest 
  is 
  about 
  

   2-70 
  (Li). 
  This 
  is 
  like 
  a 
  partly 
  dehydrated 
  fibrous 
  goe- 
  

   thite. 
  (Are 
  the 
  quartz 
  veins 
  in 
  shrinking 
  cracks 
  caused 
  

   by 
  dehydration 
  ? 
  ) 
  

  

  Cumberland, 
  England. 
  63 
  — 
  Like 
  the 
  specimen 
  from 
  Ger- 
  

   many, 
  but 
  is 
  covered 
  with 
  and 
  contains 
  many 
  minute 
  vein- 
  

   lets 
  filled 
  with 
  specular 
  hematite. 
  

  

  Recapitulation. 
  

  

  The 
  work 
  of 
  the 
  foregoing 
  pages 
  proves 
  rather 
  conclu- 
  

   sively 
  that 
  no 
  series 
  of 
  hydrates 
  of 
  ferric 
  oxide 
  exists 
  

   among 
  the 
  natural 
  minerals. 
  The 
  only 
  existing 
  hydrate 
  

   is 
  ferric 
  oxide 
  monohydrate. 
  This 
  substance 
  occurs 
  in 
  

   nature 
  in 
  two 
  polymorphic 
  forms 
  — 
  goethite 
  and 
  lepido- 
  

   crocite, 
  and 
  in 
  an 
  "amorphous" 
  condition 
  — 
  limonite. 
  

   The 
  two 
  crystallized 
  forms 
  are 
  contrasted 
  as 
  follows 
  : 
  

  

  Goethite. 
  — 
  Orthorhombic, 
  a:b:c 
  — 
  0-91: 
  1: 
  0-602; 
  den- 
  

   sity 
  (grams 
  per 
  cc.) 
  4-28 
  ±0-01; 
  a 
  — 
  2-26, 
  £ 
  = 
  2-394, 
  

   7 
  = 
  2-400; 
  streak, 
  dull 
  orange-yellow; 
  pleochroism 
  faint. 
  

  

  When 
  crystallized 
  in 
  dense 
  aggregates 
  of 
  thin 
  blades 
  

   and 
  fibers 
  enclosing 
  much 
  adsorbed 
  and 
  capillary 
  water 
  

   it 
  has 
  commonly 
  been 
  called 
  limonite 
  ; 
  however, 
  sufficient 
  

   proof 
  is 
  now 
  given 
  to 
  show 
  that 
  such 
  crystallized 
  material 
  

   is 
  really 
  goethite. 
  

  

  Lepidocrocite. 
  — 
  Orthorhombic, 
  a:b 
  :c 
  = 
  0-43 
  : 
  1 
  : 
  0-64 
  ; 
  

   density 
  (grams 
  per 
  cc.) 
  4-09 
  ± 
  0-04; 
  a 
  = 
  1-94, 
  p 
  = 
  2-20, 
  

   y 
  = 
  2-51 
  ; 
  streak, 
  dull 
  orange 
  ; 
  pleochroism 
  very 
  strong. 
  

  

  The 
  name 
  limonite 
  is 
  retained 
  for 
  material 
  which 
  

   appears 
  to 
  be 
  essentially 
  isotropic 
  ferric 
  oxide 
  mono- 
  

   hydrate 
  with 
  adsorbed 
  and 
  capillary 
  water. 
  However, 
  

   this 
  substance 
  should 
  not 
  be 
  considered 
  a 
  distinct 
  form 
  

   of 
  ferric 
  oxide 
  monohydrate, 
  as 
  the 
  real 
  nature 
  of 
  such 
  

   "amorphous" 
  substances 
  is 
  still 
  uncertain. 
  

  

  The 
  fibrous 
  mineral 
  turgite 
  is 
  variable 
  in 
  composition 
  

   and 
  considerable 
  evidence 
  is 
  given 
  that 
  it 
  probably 
  repre- 
  

   sents 
  solid 
  solutions 
  of 
  goethite 
  with 
  hematite 
  together 
  

   with 
  enclosed 
  and 
  adsorbed 
  water. 
  

  

  The 
  genetic 
  conditions 
  of 
  the 
  hydrated 
  ferric 
  oxides, 
  

   and 
  the 
  stability 
  relation 
  of 
  the 
  two 
  monohydrates, 
  are 
  

   unknown. 
  

  

  63 
  No. 
  1027 
  of 
  E. 
  B. 
  Sosman 
  and 
  J. 
  C. 
  Hostetter 
  in 
  Trans. 
  Am. 
  Inst. 
  

   Mining 
  Engineers, 
  58, 
  427, 
  1917. 
  

  

  