﻿J. 
  B. 
  Umpleby, 
  etc. 
  — 
  Custerite 
  : 
  A 
  New 
  Mineral. 
  387 
  

  

  tributed, 
  considerable 
  areas 
  of 
  some 
  of 
  the 
  crystals 
  being 
  

   nntwinned. 
  Sections 
  cut 
  normal 
  to 
  the 
  obtuse 
  bisectrix 
  show 
  

   the 
  twin 
  lamellae 
  which 
  extinguish 
  symmetrically 
  at 
  6°-7° 
  

   from 
  composition 
  plane 
  and 
  the 
  principal 
  cleavage 
  ; 
  those 
  par- 
  

   allel 
  to 
  the 
  plane 
  of 
  the 
  optic 
  axis 
  are 
  also 
  normal 
  to 
  the 
  twin 
  

   lamellae 
  but 
  the 
  extinction 
  is 
  parallel. 
  Sections 
  cut 
  normal 
  to 
  

   the 
  acute 
  bisectrix 
  show 
  no 
  twin 
  lamellae. 
  Here 
  the 
  cleavage 
  

   intersects 
  at 
  angles 
  of 
  approximately 
  90° 
  and 
  the 
  plane 
  of 
  

   extinction 
  is 
  diagonally 
  across 
  the 
  squares, 
  or 
  at 
  about 
  45° 
  to 
  

   each 
  cleavage 
  direction. 
  

  

  From 
  the 
  above 
  relations 
  of 
  the 
  optic 
  properties 
  to 
  the 
  

   cleavage 
  directions 
  and 
  twinning 
  it 
  appears 
  that 
  the 
  mineral 
  

   possesses 
  two 
  cleavages, 
  namely 
  (110) 
  and 
  (001), 
  with 
  the 
  twin- 
  

   ning 
  plane 
  parallel 
  to 
  the 
  latter. 
  The 
  conclusion 
  that 
  the 
  min- 
  

   eral 
  is 
  monoclinic 
  also 
  seems 
  warranted 
  since 
  the 
  most 
  careful 
  

   measurements 
  failed 
  to 
  show 
  an 
  inclined 
  extinction 
  of 
  the 
  

   lamellae 
  in 
  one 
  of 
  the 
  two 
  principal 
  sections 
  normal 
  to 
  them. 
  

   The 
  accuracy 
  of 
  this 
  observation 
  was 
  confirmed 
  with 
  the 
  aid 
  of 
  

   the 
  Federow 
  stage 
  on 
  which 
  a 
  crystal 
  was 
  rotated 
  in 
  the 
  plane 
  

   of 
  the 
  twinning 
  lamellae 
  and 
  extinction 
  angles 
  were 
  measured. 
  

  

  The 
  positive 
  acute 
  bisectrix 
  (Z) 
  is 
  nearly 
  normal 
  to 
  the 
  twin- 
  

   ning 
  lamellae. 
  The 
  obtuse 
  bisectrix 
  (X) 
  emerges 
  from 
  those 
  

   sections 
  cut 
  normal 
  to 
  the 
  twinning 
  lamellae 
  which 
  show 
  an 
  

   extinction 
  (Y^a) 
  of 
  6°-7°. 
  This 
  section 
  is 
  therefore 
  normal 
  

   to 
  the 
  crystallography 
  axis, 
  b. 
  

  

  The 
  optic 
  axial 
  angle 
  measured 
  with 
  the 
  Federow 
  stage 
  in 
  

   lithium, 
  sodium, 
  and 
  thalium 
  light 
  gave 
  the 
  following 
  average 
  

   results 
  of 
  readings 
  on 
  each 
  axis 
  by 
  two 
  observers 
  : 
  

  

  Measurements 
  of 
  optic 
  angles 
  of 
  custerite. 
  Readings 
  on 
  Federow 
  stage 
  with 
  

   section 
  inclined 
  17^ 
  degrees. 
  

  

  One 
  axis 
  Other 
  axis 
  

  

  Lithium 
  light 
  32*25 
  34'7 
  

  

  Sodium 
  " 
  _ 
  _. 
  31-85 
  34'65 
  

  

  Thalium 
  " 
  __. 
  31-25 
  35- 
  

  

  Correcting 
  these 
  readings, 
  which 
  were 
  checked 
  on 
  another 
  sec- 
  

   tion, 
  for 
  the 
  index 
  of 
  the 
  glass 
  and 
  the 
  tilting 
  of 
  the 
  section, 
  

   the 
  following 
  values 
  are 
  derived 
  : 
  2 
  V 
  u 
  = 
  60'5° 
  ; 
  V 
  na 
  = 
  60-1° 
  ; 
  

   2 
  V 
  th 
  = 
  59-8°. 
  Therefore, 
  2 
  E 
  = 
  105°. 
  The 
  dispersion 
  of 
  the 
  

   optic 
  axes 
  as 
  observed 
  on 
  the 
  interference 
  figure 
  is 
  rather 
  

   strong 
  and 
  p^>v. 
  Crystals 
  of 
  custerite, 
  as 
  seen 
  in 
  thin 
  section, 
  

   are 
  commonly 
  almost 
  equidimensional, 
  though 
  there 
  seems 
  to 
  

   be 
  a 
  tendency 
  toward 
  elongation 
  coincident 
  with 
  X. 
  

  

  The 
  indices 
  of 
  refraction 
  for 
  custerite 
  were 
  determined 
  by 
  

   the 
  oil-immersion 
  method, 
  the 
  values 
  below 
  being 
  fairly 
  con- 
  

   stant 
  for 
  different 
  grains 
  : 
  

  

  