﻿art, 
  7 
  THE 
  MINERAL 
  IDDINGSITE 
  ROSS 
  AND 
  SHANNON 
  9 
  

  

  evident 
  that 
  'serpentine 
  forms 
  under 
  conditions 
  where 
  reducing 
  condi- 
  

   tions 
  prevail, 
  and 
  most 
  of 
  the 
  iron 
  remains 
  in 
  the 
  ferrous 
  condition, 
  

   while 
  iddingsite 
  forms 
  where 
  oxidizing 
  conditions 
  produce 
  ferric 
  

   iron. 
  

  

  PETROLOGY 
  OF 
  IDDINGSITE-BEARING 
  ROCKS 
  

  

  RACE 
  CREEK, 
  COLO., 
  OCCURRENCE 
  

  

  The 
  rock 
  containing 
  the 
  material 
  represented 
  by 
  analysis 
  (1) 
  is 
  

   and 
  andesite 
  of 
  basaltic 
  habit 
  of 
  late 
  Tertiary 
  age 
  collected 
  from 
  a 
  

   peak 
  at 
  the 
  headwaters 
  of 
  Race 
  Creek 
  near 
  the 
  south 
  edge 
  of 
  the 
  

   Creede 
  quadrangle, 
  Colorado, 
  and 
  about 
  11 
  miles 
  south 
  of 
  South 
  

   Fork 
  on 
  the 
  Creede 
  branch 
  of 
  the 
  Denver 
  & 
  Rio 
  Grande 
  Western 
  

   Railroad. 
  The 
  rock 
  is 
  gray 
  and 
  somewhat 
  vesicular 
  and 
  in 
  the 
  hand 
  

   specimen 
  shows 
  phenocrysts 
  of 
  oligoclase, 
  quartz, 
  and 
  iddingsite. 
  

   The 
  modal 
  composition 
  of 
  the 
  rock 
  is 
  as 
  follows 
  : 
  

  

  Mineral 
  composition 
  of 
  andesite 
  from 
  Race 
  Creek, 
  Colo. 
  

  

  Quartz 
  0.5 
  

  

  Plagioclase 
  (oligoclase) 
  60. 
  

  

  Augite 
  IS. 
  7 
  

  

  Iddingsite 
  8. 
  5 
  

  

  Olivine 
  3.8 
  

  

  Magnetite 
  S. 
  

  

  Apatite 
  0. 
  5 
  

  

  Total 
  100. 
  

  

  This 
  rock 
  contains 
  augite 
  and 
  magnetite 
  in 
  notable 
  proportions, 
  but 
  

   the 
  feldspars 
  are 
  sodic 
  and 
  like 
  the 
  Brazos 
  River 
  Rock 
  it 
  would 
  be 
  

   classed 
  as 
  an 
  andesite. 
  The 
  iddingsite 
  occurs 
  in 
  euhedral 
  pseudo- 
  

   morphs 
  after 
  olivine 
  reaching 
  3 
  millimeters 
  in 
  length 
  and 
  in 
  sub- 
  

   hedral 
  aggregates. 
  The 
  alteration 
  from 
  olivine 
  to 
  iddingsite 
  is 
  not 
  

   complete 
  in 
  all 
  grains, 
  but 
  the 
  large 
  difference 
  in 
  specific 
  gravity 
  

   of 
  olivine 
  and 
  iddingsite 
  allowed 
  its 
  elimination. 
  Part 
  of 
  the 
  ma- 
  

   terial 
  was 
  rich 
  in 
  particles 
  of 
  magnetite, 
  but 
  this 
  was 
  separated 
  

   magnetically 
  and 
  eliminated 
  before 
  analysis. 
  The 
  particles 
  of 
  

   magnetite 
  are 
  arranged 
  in 
  lines 
  parallel 
  to 
  the 
  crystal 
  axes 
  of 
  the 
  

   original 
  olivines. 
  In 
  thin 
  section 
  the 
  iddingsite 
  shows 
  very 
  distinct 
  

   open 
  shrinkage 
  cracks, 
  and 
  the 
  iddingsite 
  occupies 
  from 
  15 
  to 
  20 
  

   per 
  cent 
  less 
  volume 
  than 
  the 
  olivine 
  from 
  which 
  it 
  was 
  derived. 
  

  

  LA 
  JARA 
  CREEK, 
  COLO., 
  OCCURRENCE 
  

  

  The 
  iddingsite 
  of 
  analysis 
  (2) 
  was 
  concentrated 
  from 
  a 
  nearly 
  nor- 
  

   mal 
  olivine 
  basalt 
  of 
  late 
  Tertiary 
  age 
  collected 
  near 
  the 
  base 
  of 
  a 
  

   cliff 
  on 
  La 
  Jara 
  Creek, 
  19 
  miles 
  northwest 
  of 
  Antonnito, 
  Conejos 
  

   quadrangle, 
  Colorado. 
  

   23555—25 
  2 
  

  

  