﻿akt.7 
  THE 
  MINERAL 
  IDDINGSITE 
  — 
  ROSS 
  AND 
  SHANNON 
  5 
  

  

  fig. 
  4) 
  alteration 
  has 
  occurred 
  along 
  cracks 
  with 
  very 
  sharp 
  contacts 
  

   between 
  iddingsite 
  and 
  olivine. 
  Much 
  of 
  the 
  iddingsite 
  investigated 
  

   contains 
  very 
  small 
  grains 
  of 
  magnetite 
  and 
  other 
  spinels 
  arranged 
  

   in 
  minute 
  lines 
  parallel 
  to 
  the 
  crystallographic 
  axes 
  of 
  the 
  mineral. 
  

  

  A 
  pale 
  brown 
  or 
  yellow 
  material 
  is 
  associated 
  with 
  iddingsite 
  in 
  

   some 
  rocks 
  and 
  this 
  material 
  is 
  represented 
  by 
  analyses 
  3 
  and 
  5. 
  

   This 
  is 
  usualty 
  cryptocrystaliine 
  and 
  has 
  a 
  lower 
  index 
  of 
  refraction 
  

   than 
  normal 
  iddingsite 
  and 
  a 
  small 
  axial 
  angle 
  where 
  it 
  is 
  possible 
  

   to 
  determine 
  it. 
  It 
  forms 
  a 
  rim 
  around 
  iddingsite 
  in 
  some 
  specimens 
  

   and 
  a 
  core 
  in 
  others. 
  The 
  contact 
  between 
  the 
  two 
  types 
  of 
  material 
  

   is 
  sharp 
  in 
  some 
  specimens 
  and 
  transitional 
  in 
  others. 
  The 
  evidence 
  

   does 
  not 
  clearly 
  indicate 
  whether 
  this 
  pale 
  material 
  is 
  impure, 
  im- 
  

   perfectly 
  crystallized 
  iddingsite 
  or 
  a 
  different 
  but 
  closely 
  related 
  

   mineral. 
  

  

  Usually 
  there 
  is 
  evidence 
  that 
  there 
  was 
  a 
  very 
  marked 
  loss 
  of 
  

   volume 
  during 
  alteration 
  of 
  olivine 
  to 
  iddingsite, 
  as 
  the 
  cleavage 
  

   planes 
  (pi. 
  2, 
  fig. 
  1) 
  are 
  marked 
  by 
  widely 
  gaping 
  cracks 
  that, 
  oc- 
  

   cupy 
  10 
  to 
  20 
  per 
  cent 
  of 
  the 
  volume 
  of 
  the 
  original 
  olivine. 
  

  

  ORIGIN 
  

  

  Iddingsite 
  has 
  usually 
  been 
  described 
  as 
  a 
  weathering 
  product 
  of 
  

   olivine. 
  Its 
  origin 
  through 
  the 
  processes 
  of 
  weathering 
  can 
  not 
  be 
  

   summarily 
  rejected 
  for 
  all 
  occurrences, 
  but 
  in 
  the 
  material 
  studied 
  in 
  

   the 
  preparation 
  of 
  this 
  paper 
  an 
  origin 
  through 
  weathering 
  seems 
  to 
  

   be 
  extremely 
  improbable. 
  In 
  the 
  basaltic 
  rocks 
  of 
  southern 
  Colorado 
  

   and 
  northern 
  New 
  Mexico 
  there 
  is 
  no 
  observable 
  relation 
  between 
  

   the 
  occurrence 
  of 
  olivine 
  or 
  iddingsite 
  in 
  a 
  rock 
  and 
  the 
  amount 
  of 
  

   weathering 
  that 
  rock 
  has 
  undergone. 
  In 
  general, 
  there 
  is 
  little 
  

   weathering 
  in 
  these 
  rocks 
  and 
  iddingsite 
  occurs 
  in 
  the 
  freshest 
  of 
  

   them, 
  in 
  association 
  with 
  unaltered 
  aiigite, 
  feldspars 
  that 
  are 
  not 
  

   even 
  clouded, 
  and 
  basaltic 
  glass 
  (a 
  very 
  unstable 
  material) 
  that 
  is 
  

   unchanged. 
  It 
  has 
  been 
  observed 
  evenly 
  distributed 
  from 
  top 
  to 
  bot- 
  

   tom 
  of 
  a 
  basaltic 
  sill 
  50 
  feet 
  in 
  thickness 
  where 
  no 
  trace 
  of 
  weather- 
  

   ing 
  could 
  be 
  found. 
  Its 
  occurrence 
  bears 
  no 
  relation 
  to 
  exposure 
  of 
  

   surface, 
  proximity 
  to 
  joint 
  cracks 
  or 
  relative 
  age 
  of 
  the 
  various 
  beds. 
  

   It 
  may 
  be 
  abundant 
  in 
  one 
  flow 
  and 
  be 
  absent 
  in 
  any 
  one 
  or 
  all 
  of 
  

   either 
  higher 
  or 
  lower 
  flows 
  of 
  a 
  series. 
  Several 
  flows 
  have 
  been 
  

   identified 
  where 
  iddingsite 
  of 
  similar 
  characteristics 
  is 
  present 
  over 
  

   very 
  wide 
  areas, 
  showing 
  that 
  the 
  characteristics 
  of 
  the 
  iddingsite 
  

   are 
  inherent 
  in 
  the 
  rock. 
  

  

  In 
  rocks 
  that 
  do 
  show 
  extensive 
  alteration, 
  serpentine 
  and 
  not 
  

   iddingsite 
  has 
  developed 
  from 
  olivine. 
  Some 
  basalts 
  show 
  a 
  narrow 
  

   leached 
  zone 
  at 
  the 
  surface, 
  and 
  here 
  impure 
  amorphous 
  aggregates 
  

   of 
  hydrous 
  iron 
  oxides 
  have 
  formed 
  from 
  the 
  olivine 
  crystals 
  and 
  

   not 
  iddingsite. 
  

  

  