﻿4 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  G7 
  

  

  ancl 
  the 
  double 
  refraction 
  is 
  therefore 
  strong. 
  The 
  mean 
  index 
  of 
  refraction 
  

   (not 
  given) 
  is 
  rather 
  low. 
  The 
  distinctly 
  terminated 
  prismatic 
  sections 
  are 
  

   but 
  slightly 
  pleochroic 
  and 
  show 
  no 
  cleavage. 
  The 
  interference 
  colors 
  are 
  

   moreover 
  low. 
  In 
  general 
  they 
  give 
  a 
  distinct 
  biaxial 
  figure, 
  with 
  a 
  small 
  

   angle. 
  * 
  * 
  * 
  The 
  plane 
  of 
  the 
  optic 
  axes 
  lies 
  parallel 
  to 
  the 
  longer 
  axis 
  

   of 
  the 
  prism, 
  and 
  is, 
  therefore, 
  perpendicular 
  to 
  the 
  cleavage 
  planes. 
  * 
  * 
  * 
  

   The 
  mineral 
  was 
  ascertained 
  to 
  be 
  optically 
  negative. 
  

  

  OCCURRENCE 
  

  

  Iddingsite 
  is 
  widely 
  distributed 
  in 
  the 
  basaltic 
  rocks 
  of 
  the 
  San 
  

   Juan 
  region 
  of 
  southern 
  Colorado 
  and 
  northern 
  New 
  Mexico, 
  and, 
  

   indeed, 
  throughout 
  the 
  western 
  United 
  States. 
  Petrographic 
  studies 
  7 
  

   of 
  these 
  rocks 
  show 
  conclusively 
  that 
  the 
  red 
  or 
  red-brown 
  altera- 
  

   tion 
  product 
  of 
  olivine 
  is 
  not 
  serpentine 
  and 
  indicate 
  that 
  it 
  is 
  a 
  

   definite 
  mineral 
  as 
  suggested 
  by 
  Lawson. 
  

  

  Iddingsite 
  nearly 
  always 
  gives 
  clear 
  evidence 
  of 
  its 
  derivation 
  

   from 
  olivine, 
  since 
  the 
  outlines 
  of 
  the 
  original 
  olivine 
  crystals 
  are 
  

   often 
  beautifully 
  preserved. 
  All 
  degrees 
  of 
  alteration 
  have 
  been 
  ob- 
  

   served 
  from 
  perfect, 
  homogeneous 
  crystals 
  of 
  iddingsite 
  to 
  olivine 
  

   crystals 
  with 
  the 
  merest 
  film 
  of 
  iddingsite 
  between 
  cleavage 
  cracks. 
  

   Usually 
  the 
  outer 
  zone 
  is 
  changed 
  to 
  iddingsite 
  where 
  the 
  alteration 
  

   isj 
  incomplete, 
  but 
  in 
  one 
  large 
  group 
  of 
  rocks 
  the 
  central 
  area 
  is 
  

   usually 
  iddingsite 
  with 
  an 
  outer 
  zone 
  of 
  fresh 
  olivine. 
  The 
  manner 
  

   of 
  alteration 
  appears 
  to 
  depend 
  upon 
  some 
  property 
  inherent 
  in 
  the 
  

   original 
  olivine 
  itself, 
  which 
  allows 
  some 
  parts 
  to 
  be 
  more 
  easily 
  

   altered 
  than 
  others. 
  Much 
  of 
  the 
  iddingsite 
  seems 
  at 
  first 
  glance 
  to 
  

   be 
  fibrous, 
  and 
  it 
  has 
  been 
  so 
  described. 
  Close 
  study, 
  however, 
  shows 
  

   that 
  this 
  effect 
  in 
  ihe 
  material 
  investigated 
  is 
  the 
  result 
  of 
  minute 
  

   inclusions 
  of 
  spinels, 
  magnetite, 
  or 
  hematite. 
  High 
  magnifications 
  

   of 
  small 
  grains 
  of 
  iddingsite 
  reveal 
  a 
  clean 
  fracture 
  with 
  no 
  indica- 
  

   tion 
  of 
  fibers. 
  The 
  photomicrographs 
  in 
  Plates 
  1 
  and 
  2 
  show 
  the 
  

   relationships 
  between 
  olivine 
  and 
  iddingsite 
  in 
  a 
  number 
  of 
  different 
  

   rocks. 
  In 
  many 
  specimens 
  (pi. 
  1, 
  fig. 
  1, 
  and 
  pi. 
  2, 
  fig. 
  4), 
  there 
  is 
  an 
  

   outer 
  zone 
  of 
  iddingsite 
  surrounding 
  a 
  core 
  of 
  olivine 
  with 
  a 
  ragged 
  

   area 
  between 
  the 
  two, 
  with 
  shredlike 
  masses 
  of 
  iddingsite 
  extending 
  

   into 
  the 
  olivine. 
  In 
  other 
  specimens 
  (pi. 
  1, 
  figs. 
  3, 
  4) 
  there 
  is 
  altera- 
  

   tion 
  along 
  cracks 
  in 
  the 
  olivine 
  with 
  the 
  same 
  shredlike 
  masses 
  of 
  

   iddingsite 
  extending 
  into 
  olivine. 
  In 
  some 
  specimens 
  (pi. 
  1, 
  fig. 
  3) 
  

   there 
  is 
  an 
  outer 
  zone 
  of 
  iddingsite 
  around 
  olivine 
  with 
  a 
  sharp 
  con- 
  

   tact 
  between 
  the 
  two. 
  In 
  many 
  specimens 
  the 
  large 
  phenocrysts 
  are 
  

   completely 
  changed 
  to 
  iddingsite, 
  while 
  small 
  groundmass 
  grains 
  of 
  

   olivine 
  of 
  a 
  later 
  generation 
  show 
  little 
  alteration. 
  In 
  one 
  large 
  

   group 
  of 
  rocks 
  (pi. 
  1, 
  fig. 
  3; 
  pi. 
  2, 
  figs. 
  5, 
  6) 
  there 
  is 
  an 
  inner 
  core 
  of 
  

   iddingsite 
  surrounded 
  by 
  fresh 
  olivine. 
  In 
  some 
  specimens 
  (pi. 
  1, 
  

  

  7 
  I.ai:.i, 
  Esper 
  S., 
  Bull. 
  G70, 
  U. 
  S. 
  Gool. 
  Survey, 
  p. 
  90, 
  1921. 
  

  

  