﻿art. 
  7 
  THE 
  MINERAL 
  IDDINGSITE— 
  ROSS 
  \ 
  X 
  i 
  I 
  SHANNON 
  7 
  

  

  Ransome 
  ° 
  says: 
  

  

  It 
  (iddingsite) 
  includes 
  abundant 
  grains 
  of 
  iron 
  ores, 
  and 
  frequently 
  dark 
  

   In-own 
  microscopic 
  crystals 
  of 
  chromite 
  or 
  picotite. 
  * 
  * 
  In 
  the 
  Point 
  

  

  Bonita 
  iddingsite 
  the 
  limonitic 
  p'gment 
  is 
  entirely 
  absent. 
  

  

  The 
  writers 
  have 
  found 
  very 
  large 
  amounts 
  of 
  magnetite 
  in 
  the 
  

   iddingsite 
  from 
  Race 
  Creek. 
  Colo., 
  and 
  spinels 
  in 
  that 
  from 
  Brazos 
  

   River, 
  N. 
  Mex. 
  Small 
  amounts 
  of 
  magnetite 
  or 
  -related 
  minerals 
  

   seem 
  to 
  be 
  almost 
  always 
  associated 
  with 
  iddingsite. 
  These 
  asso- 
  

   ciated 
  minerals 
  that 
  have 
  clearly 
  developed 
  by 
  the 
  same 
  processes 
  as 
  

   iddingsite 
  contribute 
  a 
  very 
  convincing 
  line 
  of 
  evidence 
  that 
  id- 
  

   dingsite 
  is 
  not 
  the 
  result 
  of 
  ordinary 
  rock 
  weathering. 
  Weathering 
  

   would 
  produce 
  hydrous 
  iron 
  oxides 
  probably 
  in 
  the 
  form 
  of 
  limonite 
  

   and 
  would 
  be 
  very 
  unlikely 
  to 
  yield 
  magnetite 
  and 
  other 
  minerals 
  of 
  

   the 
  spinel 
  group. 
  On 
  the 
  other 
  hand 
  these 
  would 
  be 
  the 
  very 
  minerals 
  

   to 
  form 
  if 
  the 
  alteration 
  of 
  olivine 
  to 
  iddingsite 
  were 
  the 
  result 
  of 
  

   magmatic 
  or 
  deuteric 
  10 
  processes. 
  

  

  Sederholm 
  says: 
  

  

  I 
  think 
  that 
  it 
  would 
  be 
  advisable 
  to 
  discriminate 
  between 
  such 
  metasomatic 
  

   changes 
  which 
  belong 
  to 
  a 
  later 
  period 
  of 
  metamorphism, 
  i. 
  e. 
  are 
  secondary 
  in 
  

   the 
  strictest 
  sense 
  of 
  the 
  word, 
  and 
  those 
  which 
  have 
  taken 
  place 
  in 
  the 
  direct 
  

   continuation 
  of 
  the 
  consolidation 
  of 
  the 
  magma 
  of 
  the 
  rock 
  itself. 
  I 
  propose 
  

   to 
  call 
  the 
  later 
  deuteric, 
  as 
  distinct 
  from 
  secondary 
  changes. 
  

  

  This 
  strongly 
  confirms 
  the 
  evidence 
  presented 
  by 
  the 
  restriction 
  in 
  

   occurrence 
  and 
  suggests 
  that 
  iddingsite 
  is 
  a 
  deuteric 
  mineral] 
  that 
  is, 
  

   it 
  has 
  been 
  produced 
  by 
  processes 
  largely 
  inherent 
  in 
  the 
  magma 
  

   itself, 
  probably 
  brought 
  about 
  by 
  gases 
  during 
  final 
  cooling. 
  

  

  The 
  conclusion 
  that 
  iddingsite 
  is 
  a 
  deuteric 
  mineral 
  first 
  based 
  

   purely 
  on 
  petrographic 
  evidence 
  is 
  strongly 
  supported 
  by 
  the 
  chem- 
  

   ical 
  analyses. 
  The 
  ordinary 
  agents 
  of 
  weathering 
  would 
  be 
  ex- 
  

   tremely 
  unlikely 
  to 
  produce 
  an 
  homogeneous 
  crystal 
  with 
  definite 
  

   optical 
  properties 
  and 
  the 
  chemical 
  composition 
  of 
  iddingsite. 
  On 
  

   page 
  8 
  is 
  given 
  a 
  typical 
  analysis 
  of 
  iddingsite 
  and 
  the 
  analysis 
  of 
  

   an 
  olivine 
  from 
  rocks 
  of 
  the 
  same 
  region. 
  

  

  A 
  comparison 
  of 
  these 
  analyses 
  shows 
  that 
  the 
  proportion 
  of 
  

   silica 
  has 
  remained 
  nearly 
  constant, 
  a 
  little 
  aluminum 
  and 
  calcium 
  

   appear 
  to 
  have 
  been 
  added, 
  the 
  iron 
  has 
  all 
  been 
  changed 
  from 
  the 
  

   ferrous 
  to 
  the 
  ferric 
  state 
  and 
  its 
  proportion 
  has 
  greatly 
  increased, 
  

   water 
  has 
  been 
  added 
  in 
  large 
  amount, 
  and 
  magnesium 
  has 
  been 
  

   largely 
  abstracted. 
  It 
  is 
  clear 
  that 
  in 
  the 
  change 
  of 
  olivine 
  to 
  

   iddingsite 
  there 
  has 
  been 
  a 
  metasomatic 
  replacement, 
  and 
  the 
  only 
  

   stages 
  through 
  which 
  these 
  rocks 
  have 
  passed 
  where 
  forces 
  seem- 
  

   ingly 
  capable 
  of 
  performing 
  such 
  work 
  have 
  been 
  a 
  ti\e 
  are 
  those 
  

  

  8 
  Ransome, 
  Frank 
  L., 
  Univ. 
  of 
  Calif. 
  Bull, 
  of 
  Dept. 
  Geol., 
  Xo. 
  1, 
  p. 
  92, 
  1893. 
  

   111 
  Sederholm, 
  J. 
  J., 
  Com. 
  Geol. 
  de 
  Flnlande, 
  Bull. 
  No. 
  48, 
  pp. 
  141-142, 
  1916. 
  

  

  