﻿akt.7 
  THE 
  MINERAL 
  IDDINGSITE 
  — 
  ROSS 
  AND 
  SHANNON 
  3 
  

  

  not 
  lie 
  on 
  the 
  cleavage 
  plane 
  ii 
  shows 
  dichroism, 
  the 
  greatesi 
  absorption 
  taking 
  

   place 
  when 
  the 
  shorl 
  axis 
  of 
  the 
  polarizer 
  is 
  parallel 
  to 
  the 
  trace 
  of 
  the 
  

   cleavage. 
  

  

  In 
  his 
  investigation 
  of 
  iddingsite, 
  Lawson 
  5 
  made 
  qualitative 
  

  

  chemical 
  tests 
  and 
  says: 
  

  

  Chemically 
  therefore 
  iddingsite 
  is 
  a 
  hydrous 
  nonaluminous 
  silicate 
  of 
  iron, 
  

   magnesia, 
  and 
  soda. 
  * 
  * 
  * 
  The 
  extraction 
  of 
  iron 
  by 
  acids 
  without 
  decom- 
  

   position 
  of 
  the 
  mineral 
  indicates 
  that 
  a 
  considerable 
  proportion 
  of 
  that 
  ele- 
  

   ment 
  is 
  present, 
  not 
  as 
  a 
  part 
  of 
  the 
  silicate 
  molecule, 
  but 
  as 
  a 
  pigment 
  in 
  the 
  

   form 
  of 
  hematite 
  or 
  limonite. 
  probably 
  the 
  latter. 
  

  

  Of 
  the 
  optical 
  properties 
  Lawson 
  says: 
  

  

  Under 
  the 
  microscope 
  the 
  cleavage 
  plates 
  prove 
  to 
  be 
  biaxial, 
  and 
  yield 
  with 
  

   great 
  definitenoss 
  a 
  figure 
  which 
  shows 
  that 
  the 
  plane 
  of 
  the 
  optic 
  axis 
  is 
  at 
  

   right 
  angles 
  to 
  the 
  cleavage 
  and 
  parallel 
  to 
  the 
  c 
  axis, 
  and 
  that 
  the 
  acute 
  

   bisectrix 
  is 
  perpendicular 
  to 
  the 
  cleavage, 
  being 
  coincident 
  with 
  the 
  a 
  axis. 
  

   In 
  these 
  plates 
  and 
  in 
  all 
  sections 
  transverse 
  to 
  the 
  cleavage 
  in 
  the 
  slides 
  the 
  

   extinction 
  is 
  strictly 
  parallel 
  to 
  the 
  cleavage, 
  to 
  the 
  fibrous 
  structure, 
  and 
  to 
  

   the 
  trace 
  of 
  the 
  pinacoids. 
  This 
  shows 
  that 
  the 
  three 
  axes 
  of 
  elasticity 
  are 
  

   parallel 
  to 
  the 
  three 
  crystallographic 
  axes, 
  respectively, 
  and 
  that 
  the 
  mineral 
  

   is 
  therefore 
  orthorhombic. 
  * 
  * 
  * 
  In 
  thin 
  section 
  iddingsite 
  becomes 
  trans- 
  

   parent 
  in 
  colors 
  which 
  range 
  from 
  a 
  deep 
  chestnut 
  brown 
  to 
  citron 
  yellow, 
  or 
  

   occasionally 
  a 
  clear 
  greenish 
  yellow. 
  The 
  pleochroism 
  is 
  strongly 
  marked 
  in 
  

   sections 
  transverse 
  to 
  the 
  cleavage, 
  particularly 
  so 
  in 
  those 
  parallel 
  to 
  the 
  

   axial 
  plane, 
  but 
  usually 
  very 
  feeble 
  in 
  sections 
  parallel 
  to 
  the 
  cleavage. 
  The 
  

   absorption 
  formula 
  is 
  c>fa>a. 
  

  

  The 
  double 
  refraction 
  (not 
  given) 
  low. 
  The 
  other 
  properties 
  

   determined 
  by 
  Lawson 
  may 
  be 
  summarized 
  as 
  follows 
  : 
  Hardness 
  

   2.4; 
  Specific 
  gravity 
  variable, 
  maximum 
  2.893; 
  Infusible 
  before 
  the 
  

   blowpipe, 
  and 
  not 
  perceptibly 
  altered. 
  Yields 
  Avater 
  in 
  the 
  closed 
  

   tube. 
  He 
  concludes 
  : 
  

  

  It 
  is 
  evidently 
  not 
  the 
  form 
  of 
  crystallized 
  serpentine 
  thermophyllite, 
  since 
  

   it 
  differs 
  from 
  the 
  latter 
  in 
  physical 
  appearance, 
  in 
  behavior 
  before 
  the 
  blow- 
  

   pipe, 
  in 
  density, 
  in 
  luster, 
  and 
  in 
  color 
  : 
  neither 
  does 
  it 
  correspond 
  optically 
  

   with 
  serpentine. 
  Moreover, 
  the 
  development 
  of 
  serpentine 
  from 
  olivine 
  by 
  

   hydration 
  is 
  accompanied 
  by 
  a 
  swelling 
  of 
  the 
  mass. 
  In 
  the 
  case 
  of 
  iddingsite, 
  

   on 
  the 
  contrary, 
  there 
  is 
  very 
  frequently 
  excellent 
  evidence 
  of 
  shrinkage. 
  

   * 
  * 
  * 
  There 
  appears 
  to 
  be 
  no 
  good 
  reason 
  for 
  regarding 
  the 
  mineral 
  as 
  a 
  

   crystallized 
  variety 
  of 
  serpentine. 
  

  

  Ivansome 
  6 
  studied 
  iddingsite 
  in 
  the 
  eruptive 
  rocks 
  oj 
  Point 
  Bonita 
  

   and 
  has 
  the 
  following 
  to 
  say 
  of 
  the 
  mineral 
  : 
  

  

  Iddingsite 
  is 
  present 
  in 
  many 
  of 
  the 
  slides 
  of 
  the 
  diabase, 
  in 
  rounded 
  ideo- 
  

   morphic 
  crystals 
  of 
  various 
  sizes 
  up 
  to 
  about 
  2 
  millimeters 
  in 
  length, 
  whose 
  

   outlines 
  are 
  strongly 
  suggestive 
  of 
  olivine. 
  The 
  color 
  varies 
  from 
  light 
  green- 
  

   ish 
  yellow 
  to 
  dark 
  dingy 
  green. 
  * 
  * 
  * 
  These 
  sections 
  are 
  pleochroic, 
  being 
  

   dark 
  yellowish 
  green 
  parallel 
  to 
  the 
  cleavage, 
  and 
  light 
  greenish 
  yellow 
  at 
  

   right 
  angles 
  to 
  that 
  position. 
  Under 
  crossed 
  nicols 
  the 
  undecomposed 
  portions 
  

   show 
  brilliant 
  mottled 
  polarization 
  colors, 
  crimson 
  and 
  green 
  predominating. 
  

  

  5 
  Lawson, 
  Andrew 
  C, 
  Univ. 
  of 
  Calif. 
  Bull.. 
  Dept. 
  of 
  Geol., 
  No. 
  1. 
  pp. 
  31-36, 
  L893. 
  

   •Ransoine, 
  F. 
  L„ 
  Bull. 
  Kept. 
  Geol., 
  t'niv. 
  Calif., 
  No. 
  1, 
  pp. 
  90-!>l:, 
  1894. 
  

  

  