﻿388 
  J. 
  B. 
  Umpleby, 
  etc. 
  — 
  Custerite 
  : 
  A 
  New 
  Mineral. 
  

  

  a 
  = 
  1*586 
  ± 
  "005 
  y 
  — 
  a 
  = 
  '012 
  

  

  (3 
  = 
  1-589 
  ± 
  -005 
  y 
  - 
  (3= 
  -009 
  

  

  y 
  = 
  1-598 
  ± 
  -005 
  fi 
  — 
  a 
  = 
  '003 
  

  

  The 
  birefringence 
  values 
  derived 
  by 
  differences 
  of 
  refrin- 
  

   gence 
  closely 
  check 
  with 
  the 
  following 
  more 
  accurate 
  direct 
  

   measurements 
  on 
  orientated 
  sections 
  respectively 
  0*06 
  and 
  0*1 
  

   of 
  a 
  millimeter 
  thick 
  : 
  

  

  y 
  — 
  a 
  = 
  -011-1- 
  

   y 
  — 
  /3 
  = 
  -009 
  + 
  

   (3 
  — 
  a 
  = 
  -004 
  — 
  

  

  Custerite 
  is 
  characterized 
  microscopically 
  by 
  its 
  moderate 
  

   index 
  of 
  refraction, 
  low 
  birefringence, 
  polysynthetic 
  twinning, 
  

   maximum 
  extinction 
  angle 
  of 
  twin 
  lamellae 
  of 
  6°-7°, 
  positive 
  

   optical 
  character, 
  distinct 
  dispersion 
  of 
  the 
  optic 
  axes 
  with 
  /><C 
  y 
  > 
  

   and 
  three 
  cleavages 
  which 
  intersect 
  at 
  high 
  angles. 
  There 
  is 
  

   no 
  known 
  mineral 
  species 
  from 
  which 
  it 
  may 
  not 
  be 
  readily 
  

   distinguished 
  optically. 
  Its 
  birefringence 
  and 
  twinning 
  sug- 
  

   gest 
  albite, 
  but 
  its 
  refringence 
  is 
  much 
  higher. 
  Its 
  relief, 
  lack 
  

   of 
  color, 
  and 
  cleavage 
  in 
  thin 
  section 
  might 
  at 
  first 
  glance 
  lead 
  to 
  

   its 
  being 
  mistaken 
  for 
  a 
  colorless 
  pyroxene, 
  but 
  its 
  refringence 
  

   and 
  birefringence 
  are 
  lower, 
  and 
  its 
  extinction 
  angle 
  is 
  less 
  than 
  

   in 
  all 
  pyroxenes 
  except 
  segirite. 
  It 
  resembles 
  hillebrandite 
  in 
  

   index 
  of 
  refraction 
  and 
  birefringence, 
  but 
  that 
  mineral 
  is 
  ortho- 
  

   rhombic, 
  optically 
  negative, 
  and 
  occurs 
  in 
  fibrous 
  forms. 
  In 
  

   relief 
  and 
  birefringence 
  also 
  it 
  resembles 
  eudialyte, 
  which, 
  how- 
  

   ever, 
  is 
  uniaxial 
  and 
  occurs 
  in 
  association 
  with 
  soda 
  minerals. 
  

  

  It 
  is 
  believed 
  that 
  custerite 
  is 
  a 
  mineral 
  of 
  rare 
  occurrence, 
  

   for 
  its 
  optical 
  properties 
  are 
  so 
  distinctive 
  that 
  it 
  would 
  scarcely 
  

   have 
  been 
  overlooked. 
  In 
  the 
  hand 
  specimen, 
  however, 
  it, 
  is 
  

   so 
  unpromising 
  in 
  appearance 
  that 
  thin 
  sections 
  of 
  it 
  may 
  never 
  

   have 
  been 
  cut. 
  The 
  mineral 
  should 
  be 
  looked 
  for 
  in 
  fluorine- 
  

   bearing 
  contact 
  zones, 
  apparently 
  in 
  the 
  border 
  phases 
  of 
  the 
  

   metamorphism. 
  

  

  Chemical 
  Properties. 
  

  

  Pyrognostics, 
  etc. 
  — 
  Heated 
  gently 
  in 
  a 
  closed 
  tube, 
  custerite 
  

   turns 
  a 
  transitory 
  yellow 
  and 
  phosphoresces 
  with 
  a 
  golden 
  yel- 
  

   low 
  light. 
  As 
  seen 
  in 
  a 
  darkened 
  room, 
  the 
  color 
  of 
  the 
  glow 
  

   is 
  like 
  that 
  of 
  a 
  deep 
  colored 
  golden 
  beryl. 
  On 
  increasing 
  the 
  

   heat 
  the 
  phosphorescence 
  is 
  destroyed 
  and 
  water 
  is 
  given 
  off. 
  

   The 
  mineral 
  does 
  not 
  decrepitate. 
  A 
  white 
  ring, 
  due 
  to 
  the 
  

   fluorine, 
  is 
  obtained 
  by 
  heating 
  the 
  mineral 
  at 
  a 
  temperature 
  suf- 
  

   ficient 
  to 
  melt 
  the 
  glass 
  tube. 
  In 
  the 
  blowpipe 
  flame, 
  custerite 
  

   fuses 
  with 
  difficulty, 
  to 
  an 
  opaque 
  white 
  enamel. 
  

  

  The 
  mineral 
  is 
  very 
  readily 
  decomposed 
  by 
  acids, 
  gelatinous 
  

   silica 
  separating 
  so 
  quickly 
  when 
  the 
  powdered 
  mineral 
  is 
  

  

  