﻿akt. 
  7 
  THE 
  MINERAL 
  IDDINGSITE 
  ROSS 
  AND 
  SHANNON 
  13 
  

  

  (010) 
  can 
  easily 
  be 
  recognized, 
  and 
  (101) 
  is 
  seen 
  less 
  frequently. 
  

   The 
  indices 
  of 
  refraction 
  are 
  : 
  

  

  a=1.792±0.003 
  (3 
  is 
  variable, 
  1.827 
  to 
  1.840+0.003 
  

   Y=1.861±0.003 
  a- 
  T 
  =0.072 
  

   The 
  axial 
  angle 
  is 
  variable, 
  the 
  extreme 
  values 
  of 
  2V 
  ranging 
  from 
  

   60 
  to 
  90°, 
  but 
  most 
  of 
  the 
  grains 
  have 
  2V=80 
  to 
  90°. 
  The 
  optical 
  

   character 
  is 
  usually 
  negative, 
  but 
  in 
  some 
  of 
  the 
  grains 
  the 
  optical 
  

   angle 
  passes 
  through 
  90° 
  and 
  the 
  mineral 
  becomes 
  positive. 
  The 
  

   dispersion 
  is 
  strong: 
  p<u 
  "when 
  the 
  character 
  is 
  negative 
  and 
  p>u 
  

   when 
  it 
  is 
  positive. 
  The 
  color 
  is 
  deep 
  reddish 
  brown 
  to 
  brownish- 
  

   ruby 
  red, 
  and 
  the 
  pleochroism 
  is 
  distinct 
  in 
  all 
  but 
  basal 
  sections. 
  

   The 
  indices 
  of 
  refraction 
  of 
  the 
  iddingsite 
  from 
  the 
  Brazos 
  River 
  

   rock 
  are 
  rather 
  high 
  but 
  the 
  other 
  optical 
  properties 
  are 
  similar 
  to 
  

   those 
  of 
  other 
  occurrences. 
  

  

  The 
  iddingsite 
  from 
  Race 
  Creek, 
  Colo., 
  is 
  brittle. 
  The 
  hardness 
  

   is 
  about 
  3.2, 
  and 
  the 
  specific 
  gravity 
  about 
  2.54. 
  In 
  small 
  grains 
  the 
  

   color 
  is 
  dull 
  dark 
  brown. 
  Three 
  mutually 
  perpendicular 
  cleavages 
  

   (100), 
  (001), 
  and 
  (010) 
  are 
  very 
  good 
  and 
  a 
  less 
  perfect 
  one 
  (101) 
  

   gives 
  plates 
  that 
  are 
  nearly 
  perpendicular 
  to 
  an 
  optic 
  axis. 
  X=a 
  

   is 
  the 
  acute 
  bisectrix. 
  The 
  indices 
  of 
  refraction 
  are 
  somewhat 
  

   variable 
  a=1.608±.005 
  £=1.646 
  ±.005, 
  y=1.655±.005 
  a~Y=.047 
  

   2Y=20°— 
  50°, 
  but 
  a 
  large 
  proportion 
  of 
  the 
  grains 
  have 
  2V= 
  

   35°— 
  42°, 
  and 
  only 
  a 
  few 
  reach 
  the 
  maximum 
  values 
  given. 
  The 
  

   optical 
  character 
  is 
  negative. 
  The 
  dispersion 
  is 
  strong 
  p<u. 
  In 
  

   thin 
  section 
  the 
  color 
  is 
  golden 
  yellow 
  to 
  golden 
  brown, 
  pleochroism 
  

   slight. 
  

  

  The 
  iddingsite 
  from 
  Bernards 
  Ferry, 
  Silver 
  City 
  quadrangle, 
  

   Idaho, 
  is 
  deep 
  reddish 
  brown 
  in 
  color. 
  It 
  has 
  very 
  perfect 
  cleavage, 
  

   but 
  the 
  great 
  brittleness 
  prevents 
  a 
  good 
  development 
  of 
  the 
  cleav- 
  

   age. 
  Cleavages 
  parallel 
  to 
  the 
  planes 
  of 
  the 
  three 
  crystal 
  axes 
  are 
  

   well 
  developed, 
  and 
  the 
  large 
  number 
  of 
  plates 
  approximately 
  per- 
  

   pendicular 
  to 
  an 
  optic 
  axis 
  indicate 
  a 
  fourth. 
  Cleavages 
  (100), 
  

   (001), 
  (010), 
  and 
  (101). 
  Optical 
  orientation 
  X=«, 
  Y=b, 
  Z=c. 
  

  

  The 
  indices 
  of 
  refraction 
  are: 
  a=1.710±.005, 
  £=1.722±.005, 
  

   7=1.754±:.005, 
  a— 
  yr=.044. 
  The 
  optical 
  angle 
  is 
  variable 
  2V=20° 
  

   to 
  65°, 
  most 
  of 
  the 
  grains 
  about 
  50°. 
  The 
  optical 
  character 
  is 
  nega- 
  

   tive, 
  dispersion 
  strong. 
  The 
  color 
  is 
  deep 
  brownish 
  red, 
  pleochro- 
  

   ism 
  slight 
  

  

  For 
  purposes 
  of 
  comparison 
  the 
  optical 
  properties 
  of 
  iddingsite 
  

   from 
  other 
  localities 
  are 
  given 
  below 
  : 
  

  

  1. 
  Type 
  material 
  from 
  carmeloite, 
  Carmelo 
  Bay, 
  Calif.; 
  reddish 
  

   brown; 
  extinction 
  parallel 
  to 
  cleavage; 
  X 
  is 
  normal 
  to 
  cleavage 
  

   plates. 
  Optically 
  negative. 
  Dispersion 
  p<v 
  (strong). 
  Pleochroic. 
  

   a=1.723±.003. 
  p=1.745±.003. 
  y=1.765±.003. 
  a— 
  T 
  =0.42. 
  2V 
  large. 
  

  

  