﻿290 
  Larsen 
  and 
  Hunt 
  — 
  Vanadiferous 
  ^Egirites. 
  

  

  foot 
  or 
  more. 
  In 
  the 
  characteristic 
  alteration 
  the 
  apatite, 
  

   ore, 
  and 
  biotite 
  are 
  not 
  changed, 
  but 
  the 
  pyroxene 
  is 
  altered 
  to 
  

   a 
  fibrous 
  aggregate 
  of 
  amphi 
  boles 
  with 
  calcite 
  and 
  sulphides. 
  

   The 
  amphibole 
  has 
  unusual 
  and 
  somewhat 
  variable 
  optical 
  pro- 
  

   perties. 
  Some 
  of 
  the 
  deeper-colored 
  parts 
  resemble 
  glauco- 
  

   phane, 
  but 
  the 
  most 
  common 
  type 
  has 
  the 
  following 
  optical 
  

   properties: 
  The 
  indices 
  of 
  refraction 
  are 
  a 
  = 
  1*620 
  ± 
  0*003 
  

   y 
  = 
  1-630 
  ± 
  0*003. 
  The 
  birefringence 
  is 
  rather 
  w 
  T 
  eak. 
  Sec- 
  

   tions 
  cut 
  parallel 
  to 
  the 
  side 
  pinacoid 
  do 
  not 
  extinguish 
  in 
  

   white 
  light, 
  but 
  on 
  revolving 
  on 
  the 
  stage 
  of 
  the 
  microscope 
  

   they 
  show 
  a 
  succession 
  of 
  abnormal 
  interference 
  colors. 
  In 
  

   monochromatic 
  light 
  this 
  section 
  showed 
  the 
  following 
  extinc- 
  

   tion 
  angles: 
  

  

  For 
  Li-light 
  X 
  ^ 
  c 
  = 
  45° 
  

   " 
  Na 
  " 
  X 
  xs 
  c 
  = 
  41° 
  

   " 
  Th 
  " 
  X 
  A 
  c 
  = 
  35° 
  

  

  Z 
  is 
  parallel 
  to 
  c. 
  The 
  pleochroism 
  is 
  moderate 
  : 
  Z 
  == 
  pale-blue 
  or 
  

   pale-violet, 
  Y 
  == 
  pale-yellowish, 
  X 
  = 
  pale-green. 
  

  

  Vanadiferous 
  JEgikite. 
  

  

  The 
  vanadiferous 
  segirite 
  occurs 
  chiefly 
  near 
  the 
  borders 
  of 
  

   the 
  veins, 
  but 
  it 
  is 
  not 
  entirely 
  confined 
  to 
  the 
  borders. 
  It 
  

   commonly 
  is 
  in 
  rather 
  well-developed 
  acicular 
  crystals 
  or 
  

   fibrous 
  aggregates 
  oriented 
  nearly 
  normal 
  to 
  the 
  walls 
  of 
  the 
  

   vein. 
  In 
  other 
  specimens 
  beautiful 
  spherulites 
  of 
  the 
  mineral, 
  

   up 
  to 
  an 
  inch 
  across, 
  are 
  scattered 
  rather 
  thickly 
  in 
  the 
  vein 
  

   material. 
  These 
  spherulites 
  are 
  made 
  up 
  of 
  well-formed 
  

   acicular 
  crystals 
  of 
  the 
  segirite 
  about 
  a 
  millimeter 
  in 
  cross 
  sec- 
  

   tion 
  radiating 
  from 
  a 
  common 
  center 
  and 
  piercing 
  the 
  other 
  

   minerals. 
  One 
  specimen 
  shows 
  fairly 
  well-formed 
  crystals 
  

   several 
  millimeters 
  in 
  diameter. 
  The 
  tendency 
  of 
  the 
  segirite 
  

   to 
  form 
  well-bounded, 
  prismatic 
  crystals 
  is 
  marked. 
  The 
  thin 
  

   sections 
  show 
  that 
  the 
  segirite 
  pierces 
  the 
  other 
  minerals 
  of 
  

   the 
  veins 
  and 
  especially 
  the 
  quartz 
  ; 
  one 
  individual 
  may 
  cross 
  

   several 
  quartz 
  grains. 
  Such 
  crystals 
  are 
  commonly 
  skeleton- 
  

   like 
  with 
  a 
  more 
  or 
  less 
  continuous 
  shell 
  and 
  several 
  ragged 
  

   strips 
  between. 
  Nearly 
  all 
  of 
  the 
  individuals 
  show 
  intergrown 
  

   quartz 
  ; 
  a 
  few 
  show, 
  in 
  addition, 
  included 
  sulphides. 
  In 
  other 
  

   respects 
  they 
  are 
  clear 
  and 
  free 
  from 
  inclusions. 
  

  

  Crystallography. 
  — 
  Although 
  the 
  bulk 
  of 
  the 
  material 
  is 
  

   unsuitable 
  for 
  accurate 
  goniometric 
  measurements, 
  after 
  a 
  

   careful 
  search 
  about 
  two 
  dozen 
  more 
  or 
  less 
  perfectly 
  developed 
  

   crystals 
  were 
  isolated 
  and 
  their 
  examination 
  led 
  to 
  the 
  iden- 
  

   tification 
  of 
  six 
  forms 
  of 
  simple 
  indices. 
  Crystals 
  of 
  two 
  dis- 
  

   tinct 
  types 
  of 
  development 
  are 
  represented. 
  Figure 
  1 
  illustrates 
  

   the 
  short 
  prismatic 
  type 
  with 
  the 
  positive 
  hemi-orthodome 
  as 
  

  

  