﻿12 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  grained 
  andesite, 
  is 
  light 
  gray 
  in 
  color, 
  and 
  is 
  very 
  fresh, 
  showing 
  

   no 
  indications 
  of 
  weathering. 
  Its 
  modal 
  composition 
  is 
  as 
  follows 
  : 
  

  

  Mineral 
  composition 
  of 
  andesite 
  from 
  Brazos 
  River, 
  N. 
  Mex. 
  

  

  Plagioclase 
  (AbeoAn^) 
  82. 
  9 
  

  

  Augite 
  8. 
  7 
  

  

  Iddingsite 
  4.6 
  

  

  Magnetite 
  3. 
  8 
  

  

  Total 
  100. 
  

  

  This 
  rock 
  contains 
  rather 
  sodic 
  feldspars 
  and 
  is 
  unusually 
  low 
  in 
  

   femic 
  minerals 
  to 
  be 
  olivine 
  bearing, 
  but 
  the 
  form 
  of 
  the 
  red 
  altera- 
  

   tion 
  product 
  shows 
  that 
  it 
  was 
  derived 
  from 
  that 
  mineral, 
  and 
  other 
  

   specimens 
  from 
  the 
  same 
  horizon 
  in 
  the 
  region 
  show 
  olivine 
  in 
  all 
  

   degrees 
  of 
  alteration 
  to 
  iddingsite. 
  The 
  alteration 
  mineral 
  occurs 
  in 
  

   irregularly 
  rounded 
  grains, 
  in 
  aggregates 
  of 
  several 
  grains, 
  and 
  as 
  

   perfectly 
  bounded, 
  pseudomorphs 
  after 
  olivine, 
  varying 
  in 
  size 
  from 
  

   0.05 
  to 
  2 
  millimeters. 
  Many 
  of 
  the 
  grains 
  contain 
  minute 
  particles 
  

   distributed 
  in 
  lines 
  that 
  run 
  parallel 
  to 
  one 
  of 
  the 
  crystallographic 
  

   axes 
  of 
  the 
  original 
  olivine. 
  In 
  partty 
  altered 
  olivine, 
  between 
  id- 
  

   dingsite 
  and 
  colorless 
  olivine, 
  may 
  be 
  seen 
  a 
  brown 
  zone 
  which 
  is 
  

   filled 
  with 
  lines 
  of 
  inclusions 
  that 
  continue 
  into 
  homogeneous 
  id- 
  

   dingsite. 
  These 
  inclusions 
  are 
  dark 
  brown 
  in 
  color 
  and 
  very 
  small. 
  

   They 
  are 
  isotropic 
  and 
  have 
  an 
  index 
  of 
  refraction 
  a 
  little 
  lower 
  than 
  

   1.74. 
  Their 
  specific 
  gravity 
  is 
  greater 
  than 
  that 
  of 
  iddingsite, 
  since 
  

   it 
  was 
  possible 
  to 
  separate 
  and 
  reject 
  that 
  part 
  of 
  the 
  iddingsite 
  con- 
  

   taining 
  them 
  in 
  greatest 
  amount. 
  These 
  things 
  make 
  it 
  seem 
  proba- 
  

   ble 
  that 
  they 
  are 
  iron-magnesium 
  spinel. 
  

  

  PHYSICAL 
  PROPERTIES 
  

  

  Iddingsite 
  from 
  many 
  localities 
  has 
  been 
  studied, 
  but 
  the 
  material 
  

   from 
  Brazos 
  River, 
  N. 
  Mex., 
  is 
  the 
  most 
  homogeneous 
  and 
  shows 
  the 
  

   physical 
  properties 
  in 
  greatest 
  perfection. 
  For 
  that 
  reason 
  the 
  

   Brazos 
  iddingsite 
  will 
  be 
  described 
  in 
  detail 
  and 
  that 
  from 
  other 
  

   localities 
  more 
  briefly. 
  

  

  The 
  iddingsite 
  of 
  the 
  Brazos 
  River 
  rock 
  is 
  very 
  brittle. 
  The 
  

   hardness 
  is 
  about 
  3.5 
  and 
  the 
  specific 
  gravity 
  2.80. 
  In 
  small 
  grains 
  

   the 
  cleavage 
  is 
  somewhat 
  imperfect, 
  but 
  four 
  cleavages 
  can 
  be 
  

   recognized. 
  If 
  the 
  orientation 
  X=a, 
  Y=b, 
  Z=c 
  (a 
  = 
  a, 
  b 
  = 
  &, 
  c 
  = 
  c), 
  

   proposed 
  by 
  Lawson, 
  is 
  retained, 
  there 
  is 
  one 
  cleavage 
  (100) 
  perpen- 
  

   dicular 
  to 
  the 
  acute 
  bisectrix; 
  a 
  second 
  (001) 
  is 
  perpendicular 
  to 
  the 
  

   obtuse 
  bisectrix; 
  a 
  third 
  (010) 
  is 
  parallel 
  to 
  the 
  plane 
  of 
  the 
  optic 
  

   axes; 
  and 
  the 
  fourth 
  (101) 
  is 
  nearly 
  perpendicular 
  to 
  an 
  optic 
  axis. 
  

   That 
  is, 
  a 
  cleavage 
  parallel 
  to 
  the 
  macropinacoid, 
  one 
  parallel 
  to 
  the 
  

   basal 
  pinacoid, 
  one 
  parallel 
  to 
  the 
  brachy-pinacoid, 
  and 
  one 
  parallel 
  

   to 
  the 
  macrodome. 
  In 
  thin 
  sections 
  three 
  cleavages 
  (100) 
  (001) 
  and 
  

  

  