﻿art. 
  7 
  THE 
  MINERAL 
  IDDINGSITE 
  ROSS 
  AND 
  SHANNON 
  

  

  The 
  average 
  column 
  gives 
  the 
  following 
  ratios: 
  

  

  17 
  

  

  The 
  formula 
  derived 
  from 
  the 
  ratios 
  is 
  : 
  

  

  MgO 
  . 
  Fe 
  2 
  3 
  . 
  3Si0 
  2 
  . 
  4H 
  2 
  . 
  

  

  with 
  the 
  magnesia 
  replaced 
  in 
  part 
  by 
  CaO 
  which 
  is 
  in 
  the 
  ratio, 
  

   approximately, 
  of 
  CaO 
  : 
  MgO 
  = 
  1:4. 
  The 
  calculated 
  composition 
  

   for 
  this 
  formula 
  is 
  as 
  follows 
  : 
  

  

  Si0 
  2 
  39. 
  66 
  

  

  Fe,0 
  3 
  35. 
  01 
  

  

  CaO 
  2.46 
  

  

  MgO 
  7. 
  07 
  

  

  H2O+ 
  7.90 
  

  

  H2O— 
  7. 
  90 
  

  

  Total 
  100. 
  00 
  

  

  In 
  view 
  of 
  the 
  agreement 
  of 
  the 
  above 
  analyses 
  with 
  each 
  other 
  

   and 
  with 
  the 
  theoretical 
  composition, 
  this 
  formula 
  may 
  be 
  confi- 
  

   dently 
  quoted 
  as 
  that 
  of 
  the 
  normal 
  iddingsite. 
  This 
  is 
  especially 
  

   true 
  since 
  a 
  comparison 
  of 
  optical 
  properties 
  indicates 
  that 
  the 
  above 
  

   are 
  typical 
  of 
  95 
  per 
  cent 
  of 
  all 
  iddingsites 
  studied 
  by 
  the 
  writers 
  

   or 
  reported 
  by 
  others. 
  Nearly 
  all 
  of 
  the 
  red-brown 
  material 
  sec- 
  

   ondary 
  to 
  olivine 
  is 
  shown 
  by 
  its 
  refractive 
  indices 
  and 
  other 
  prop- 
  

   erties 
  to 
  be 
  of 
  this 
  type 
  and 
  presumably 
  of 
  this 
  composition. 
  

  

  The 
  cryptocrystalline 
  materials 
  represented 
  by 
  analyses 
  3 
  and 
  5 
  

   give 
  the 
  same 
  formula 
  as 
  the 
  crystalline 
  iddingsites. 
  They 
  are 
  dis- 
  

   tinguished 
  by 
  pale 
  yellow 
  color, 
  low 
  refractive 
  indices, 
  and 
  very 
  small 
  

   extinction 
  angle. 
  Often 
  there 
  is 
  a 
  sharp 
  contact 
  between 
  the 
  idding- 
  

   site 
  and 
  the 
  cryptocrystalline 
  material 
  while 
  the 
  latter 
  grades 
  almost 
  

   imperceptibly 
  into 
  the 
  residual 
  olivine. 
  This 
  cryptocrystalline 
  ma- 
  

   terial 
  may 
  represent 
  a 
  transition 
  stage 
  in 
  the 
  alteration 
  of 
  olivine 
  

   to 
  iddingsite. 
  While 
  of 
  the 
  same 
  composition, 
  it 
  is 
  sufficiently 
  dis- 
  

   tinct 
  optically 
  to 
  suggest 
  that 
  it 
  is 
  a 
  distinct 
  mineral 
  — 
  possibly 
  a 
  

   variety 
  of 
  chloropal. 
  It 
  is 
  certainly 
  not 
  the 
  material 
  commonly 
  

   called 
  iddingsite. 
  

  

  