﻿akt,7 
  THE 
  MINERAL 
  [DDINGSITE 
  — 
  ROSS 
  AND 
  SHANNON 
  19 
  

  

  The 
  changes 
  involved 
  are 
  principally 
  abstraction 
  of 
  magnesium 
  

   oxide 
  (MgO), 
  oxidization 
  of 
  ferrous 
  oxide 
  (FeO) 
  to 
  ferric 
  oxide 
  

   (Fe 
  2 
  3 
  ) 
  and 
  addition 
  of 
  water 
  (H 
  2 
  0). 
  

  

  Iddingsite 
  has 
  a 
  composition 
  and 
  optical 
  properties 
  distinct 
  from 
  

   any 
  described 
  mineral, 
  and 
  it 
  is 
  not 
  related 
  to 
  serpentine 
  in 
  mode 
  

   of 
  origin, 
  in 
  chemical 
  composition, 
  or 
  in 
  physical 
  properties. 
  Tim.-; 
  

   it 
  appears 
  to 
  be 
  a 
  distinct 
  mineral 
  species. 
  

  

  The 
  normal 
  type 
  of 
  iddingsite 
  is 
  represented 
  by 
  the 
  formula 
  : 
  

   MgO 
  . 
  Fe 
  2 
  3 
  . 
  3Si0 
  2 
  . 
  4H 
  2 
  where 
  MgO 
  is 
  replaced 
  by 
  CaO 
  in 
  the 
  

   ratio 
  4:1, 
  and 
  varying 
  proportions 
  of 
  Fe 
  2 
  3 
  are 
  replaced 
  by 
  A1 
  2 
  3 
  . 
  

  

  EXPLANATION 
  OF 
  PLATES 
  

  

  Plate 
  1 
  

  

  Fig. 
  1. 
  Iddingsite 
  from 
  andesite 
  collected 
  at 
  the 
  headwaters 
  of 
  Race 
  Creek 
  

   near 
  the 
  south 
  border 
  of 
  Creede 
  quadrangle, 
  Colorado. 
  Open 
  cracks 
  

   show 
  the 
  loss 
  of 
  volume 
  on 
  alteration 
  of 
  olivine 
  to 
  iddingsite. 
  Altera- 
  

   tion 
  is 
  complete. 
  

  

  2. 
  Iddingsite 
  in 
  andesite 
  from 
  1 
  mile 
  west 
  of 
  Osier, 
  N. 
  Mex., 
  near 
  

  

  Colorado-New 
  Mexico 
  State 
  line. 
  Shows 
  characteristic 
  outline 
  of 
  

   olivine 
  crystals. 
  Alteration 
  nearly 
  complete. 
  

  

  3. 
  Iddingsite 
  in 
  basalt 
  from 
  Santa 
  Clara 
  Creek, 
  13 
  miles 
  west 
  of 
  Espanola, 
  

  

  N. 
  Mex. 
  Iddingsite 
  forming 
  sharp 
  outer 
  borders 
  around 
  unaltered 
  

   olivine. 
  

  

  4. 
  Andesite 
  from 
  southeast 
  flank 
  of 
  Green 
  Mountain, 
  northern 
  part 
  of 
  

  

  Conejos 
  quadrangle, 
  Colorado. 
  Euhedral 
  phenocrysts 
  of 
  olivine 
  with 
  

   very 
  narrow 
  outer 
  border 
  of 
  iddingsite. 
  

   5 
  and 
  G. 
  Basalt 
  from 
  cliff 
  on 
  north 
  side 
  of 
  Los 
  Magotes, 
  southeast 
  part 
  Conejos 
  

   quadrangle, 
  Colorado. 
  Iddingsite 
  core 
  with 
  narrow 
  sharp 
  outer 
  

   border 
  of 
  olivine. 
  

  

  Plate 
  2 
  

  

  Fig. 
  7. 
  Basalt 
  with 
  iddingsite 
  collected 
  4 
  miles 
  south 
  of 
  the 
  crest 
  of 
  San 
  An- 
  

   tonio 
  Peak 
  and 
  8 
  milts 
  north 
  of 
  Tres 
  Piedras, 
  N. 
  Mex. 
  Large 
  pheno- 
  

   crysts 
  of 
  olivine 
  with 
  core 
  of 
  iddingsite 
  and 
  outer 
  rim 
  of 
  olivine. 
  

  

  8. 
  Basalt 
  from 
  cliffs 
  on 
  north 
  side 
  of 
  Los 
  Magotes, 
  Conejos 
  quadrangle. 
  

  

  Colorado. 
  Many 
  small 
  grains 
  of 
  iddingsite 
  with 
  sharp 
  outer 
  rim 
  of 
  

   olivine. 
  Large 
  crystal 
  on 
  upper 
  border 
  shows 
  core 
  of 
  olivine. 
  

  

  9. 
  Basalt 
  from 
  mouth 
  of 
  Rito 
  de 
  los 
  Frijoles 
  Canyon, 
  10 
  miles 
  south- 
  

  

  west 
  of 
  San 
  Ildefonso, 
  N. 
  Mex. 
  Phenocryst 
  of 
  olivine 
  showing 
  altera- 
  

   tion 
  to 
  iddingsite 
  along 
  border 
  and 
  cracks. 
  

  

  10. 
  Basalt 
  dike 
  in 
  Cerro 
  Negro 
  volcanic 
  cone 
  about 
  10 
  miles 
  east 
  of 
  Tres 
  

  

  Piedras, 
  X. 
  Mex. 
  Phenocryst 
  of 
  olivine 
  with 
  very 
  narrow 
  sharp 
  

   films 
  of 
  iddingsite 
  developing 
  along 
  cracks. 
  

  

  11. 
  Iddingsite 
  in 
  Brazos 
  River 
  andesite 
  from 
  one-half 
  mile 
  east 
  of 
  Brazos 
  

  

  River, 
  N. 
  Mex., 
  about 
  15 
  miles 
  south 
  of 
  Osier, 
  Colo. 
  Iddingsite 
  

   crystal 
  showing 
  characteristic 
  outline 
  of 
  olivine. 
  Near 
  the 
  center 
  

   of 
  crystal 
  are 
  shown 
  2 
  cleavages 
  parallel 
  to 
  crystal 
  axes 
  and 
  the 
  

   cleavage 
  parallel 
  to 
  the 
  macrodome 
  (101). 
  

  

  12. 
  Basalt 
  from 
  west 
  slope 
  of 
  Mesa 
  La 
  Sauses, 
  10 
  miles 
  east 
  of 
  La 
  Jara, 
  

  

  Colo. 
  Phenocryst 
  of 
  olivine 
  entirely 
  altered 
  to 
  iddingsite. 
  

  

  o 
  

  

  