﻿Larsen 
  and 
  Hunt 
  — 
  Vanadiferous 
  ^Egirites. 
  293 
  

  

  The 
  indices 
  of 
  refraction 
  of 
  the 
  Montana 
  mineral, 
  deter- 
  

   mined 
  by 
  the 
  oil-immersion 
  method, 
  vary 
  somewhat 
  and 
  the 
  

   values 
  given 
  are 
  about 
  the 
  average. 
  The 
  birefringence 
  was 
  

   measured 
  directly 
  with 
  a 
  Babinet 
  compensator. 
  The 
  value 
  of 
  

   7-/3 
  was 
  found 
  to 
  be 
  O'Oll. 
  Direct 
  measurements 
  of 
  the 
  axial 
  

   angle 
  were 
  not 
  satisfactory, 
  but 
  from 
  the 
  values 
  of 
  the 
  bire- 
  

   fringence 
  it 
  was 
  computed 
  to 
  be 
  69° 
  ± 
  3°. 
  The 
  dispersion 
  of 
  

   the 
  optic 
  axes 
  is 
  strong 
  and 
  p>v, 
  as 
  in 
  ordinary 
  segirite. 
  The 
  

   extinction 
  on 
  the 
  side 
  pinacoid 
  gave 
  for 
  an 
  average 
  of 
  eight 
  

   measurements 
  in 
  sodium 
  light 
  1'4°, 
  for 
  an 
  average 
  of 
  ten 
  

   measurements 
  in 
  white 
  light 
  1*2°. 
  The 
  color 
  and 
  pleochroism 
  

   of 
  the 
  segirite 
  from 
  Libby 
  are 
  its 
  most 
  marked 
  characteristics. 
  

   The 
  pleochroism 
  is 
  strong 
  : 
  X 
  is 
  dark-brown, 
  Y 
  is 
  lighter- 
  

   brown, 
  and 
  Z 
  is 
  pale-yellowish 
  brown 
  or 
  amber. 
  

  

  Chemical 
  properties. 
  — 
  The 
  material 
  for 
  analyses 
  was 
  first 
  

   carefully 
  picked 
  out 
  by 
  hand 
  and 
  was 
  later 
  separated 
  from 
  the 
  

   small 
  amount 
  of 
  admixed 
  quartz, 
  microcline, 
  calcite, 
  and 
  pyrite, 
  

   by 
  a 
  weak 
  electro-magnet. 
  The 
  resulting 
  material 
  was 
  found 
  

   on 
  microscopic 
  examination 
  to 
  contain 
  a 
  very 
  small 
  amount 
  of 
  

   impurity. 
  

  

  From 
  a 
  chemical 
  standpoint 
  the 
  chief 
  interest 
  centers 
  about 
  

   the 
  relatively 
  high 
  content 
  of 
  vanadium. 
  Hillebrand* 
  has 
  

   called 
  attention 
  to 
  the 
  presence 
  of 
  vanadium 
  in 
  basic 
  igneous 
  

   rocks 
  (under 
  60 
  per 
  cent 
  Si0 
  2 
  ), 
  where 
  as 
  V" 
  2 
  3 
  it 
  replaces 
  alu- 
  

   mina 
  and 
  ferric 
  oxide 
  in 
  pyroxene, 
  hornblende, 
  and 
  biotites. 
  

   In 
  these 
  instances, 
  however, 
  its 
  presence 
  is 
  usually 
  recorded 
  as 
  

   a 
  few 
  hundredths 
  of 
  one 
  per 
  cent. 
  In 
  one 
  case, 
  that 
  of 
  a 
  bio- 
  

   tite 
  separated 
  from 
  a 
  pyroxenic 
  gneiss, 
  the 
  vanadium 
  content 
  

   was 
  somewhat 
  higher, 
  reaching 
  *13 
  per 
  cent 
  V 
  2 
  3 
  . 
  In 
  the 
  ros- 
  

   coelite 
  mica 
  the 
  content 
  of 
  V 
  2 
  3 
  is 
  much 
  higher. 
  It 
  would 
  

   seem 
  from 
  this 
  that 
  the 
  content 
  of 
  vanadium 
  in 
  mafic 
  minerals 
  

   other 
  than 
  mica 
  rarely 
  exceeds 
  two-tenths 
  of 
  one 
  per 
  cent. 
  

   The 
  two 
  minerals 
  here 
  described 
  were 
  unusual 
  in 
  this 
  respect 
  : 
  

   in 
  one 
  specimen 
  the 
  vanadium 
  oxide 
  was 
  present 
  to 
  the 
  extent 
  

   of 
  almost 
  4 
  per 
  cent, 
  while 
  the 
  second 
  showed 
  a 
  slightly 
  

   smaller 
  amount. 
  

  

  Table 
  2 
  gives 
  the 
  chemical 
  analyses 
  of 
  segirite 
  from 
  Libby, 
  

   Montana, 
  by 
  Hunt, 
  and 
  from 
  Brevig 
  by 
  Doelterjf 
  the 
  two 
  

   analyses 
  show 
  a 
  somewhat 
  close 
  similarity. 
  In 
  the 
  latter 
  case 
  

   Ti0 
  2 
  was 
  absent 
  and 
  V 
  2 
  3 
  either 
  absent 
  or 
  else 
  not 
  sought 
  for. 
  

  

  Columns 
  1 
  and 
  2 
  of 
  Table 
  3 
  give 
  the 
  molecular 
  ratios 
  cor- 
  

   responding 
  to 
  the 
  analyses 
  of 
  the 
  Montana 
  segirite. 
  If 
  we 
  

   deduct 
  from 
  this 
  the 
  equivalent 
  of 
  Na 
  2 
  O.Fe 
  2 
  3 
  .4Si0 
  2 
  (acmite) 
  

  

  * 
  Hillebrand, 
  W. 
  F. 
  : 
  The 
  analysis 
  of 
  silicate 
  and 
  carbonate 
  rocks 
  : 
  Bull. 
  

   U. 
  S. 
  Geol. 
  Survey, 
  No. 
  422, 
  pp. 
  20, 
  24, 
  149, 
  1910. 
  

  

  fDoelter, 
  Tscherm. 
  Mitth., 
  vol. 
  i, 
  p. 
  376, 
  1878; 
  also 
  in 
  Zeitschr. 
  f. 
  Kryst., 
  

   vol. 
  iv, 
  p. 
  91. 
  

  

  