﻿322 
  Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  refractive 
  indices 
  within 
  the 
  rather 
  narrow 
  limits 
  of 
  about 
  

   2-00 
  to 
  2-10 
  when 
  saturated 
  with 
  a 
  liquid 
  of 
  corresponding 
  

   refractive 
  index. 
  Such 
  material, 
  which 
  has 
  often 
  been 
  

   called 
  limonite, 
  will 
  be 
  referred 
  to 
  as 
  limonite 
  here. 
  22 
  

   Double 
  refraction 
  is 
  common 
  in 
  such 
  material, 
  usually 
  it 
  

   is 
  indefinite 
  and 
  not 
  strong, 
  but 
  occasionally 
  it 
  reaches 
  

   0-04 
  and 
  is 
  almost 
  as 
  definite 
  as 
  in 
  fibrous 
  goethites. 
  For 
  

   example, 
  in 
  the 
  cylindrical 
  crusts 
  around 
  stalactites 
  and 
  

   in 
  the 
  spheroidal 
  grains 
  the 
  ray 
  vibrating 
  along 
  the 
  radius 
  

   is 
  most 
  strongly 
  refracted 
  as 
  in 
  fibrous 
  goethite. 
  In 
  rare 
  

  

  Fig. 
  1. 
  

  

  100 
  

  

  cases 
  a 
  decided 
  tendency 
  for 
  minute 
  fragments 
  to 
  be 
  splin- 
  

   tery 
  makes 
  this 
  material 
  look 
  somewhat 
  like 
  fibrous 
  goe- 
  

   thite. 
  

  

  Lepidocrocite: 
  — 
  Optical 
  studies 
  of 
  the 
  reddish, 
  scaly 
  

   crystals 
  called 
  rubinglimmer 
  and 
  lepidocrocite, 
  led 
  

   Lacroix 
  to 
  propose 
  that 
  they 
  be 
  classed 
  together 
  as 
  lepi- 
  

   docrocite 
  and 
  separated 
  from 
  goethite, 
  with 
  which 
  they 
  

   had 
  formerly 
  been 
  identified. 
  23 
  Crystals 
  supposed 
  to 
  con- 
  

   tain 
  more 
  water 
  than 
  goethite 
  and 
  which 
  from 
  later 
  24 
  

   descriptions 
  were 
  seen 
  to 
  be 
  optically 
  similar 
  to 
  lepidoc- 
  

   rocite, 
  were 
  called 
  hydro-goethite 
  by 
  Zemjatschensky. 
  25 
  

   Crystallographic 
  measurements, 
  mostly 
  with 
  the 
  micro- 
  

  

  22 
  See 
  A 
  Review 
  of 
  the 
  Amorphous 
  Minerals, 
  A. 
  F. 
  Rogers, 
  J. 
  Geol. 
  25, 
  528. 
  

  

  23 
  Miner, 
  d. 
  France, 
  III, 
  360. 
  

  

  24 
  J. 
  Samojloff, 
  Zs. 
  Kryst. 
  34, 
  701 
  and 
  35, 
  272, 
  abs. 
  

   26 
  Zs. 
  Kryst, 
  20, 
  185 
  abs. 
  

  

  