﻿THE 
  MINERALS 
  OE 
  THE 
  MICA 
  EAMILY. 
  367 
  

  

  IV. 
  Analysis 
  of 
  a 
  Hydromuscovite 
  (Margarodite). 
  Dana. 
  

  

  Silica 
  46-50 
  

  

  Alumina 
  33*91 
  

  

  Potash 
  7-32 
  

  

  Ferric 
  Oxide 
  2-69 
  

  

  Magnesia 
  0*90 
  

  

  Soda 
  2-70 
  

  

  Fluorine 
  0-82 
  

  

  Chlorine 
  0*31 
  

  

  Water 
  4-63 
  

  

  99-78 
  

   Two 
  pieces 
  of 
  biotite 
  were 
  in 
  like 
  manner 
  exposed 
  for 
  a 
  year 
  to 
  

   the 
  action 
  of 
  water, 
  pure 
  and 
  carbonated. 
  Fine 
  mica-dust, 
  the 
  

   minute 
  flakes 
  of 
  which 
  had 
  the 
  same 
  general 
  forms 
  as 
  those 
  obtained 
  

   from 
  the 
  muscovite, 
  was 
  also 
  observed 
  at 
  the 
  bottom 
  of 
  each 
  of 
  the 
  

   vessels 
  at 
  the 
  end 
  of 
  the 
  period 
  of 
  immersion 
  ; 
  but 
  in 
  this 
  case 
  that 
  

   containing 
  the 
  carbonic-acid 
  water 
  appeared 
  to 
  have 
  a 
  distinctly 
  

   larger 
  amount 
  of 
  dust 
  than 
  the 
  vessel 
  filled 
  with 
  pure 
  water. 
  

   When 
  the 
  biotite-mica 
  dust 
  which 
  had 
  been 
  formed 
  by 
  the 
  action 
  

   of 
  the 
  pure 
  distilled 
  water 
  and 
  air 
  was 
  examined, 
  it 
  was 
  found 
  

   that 
  all 
  the 
  minute 
  plates 
  had 
  become 
  distinctly 
  hydrated, 
  had 
  been, 
  

   in 
  fact, 
  converted 
  into 
  hydrobiotites. 
  No 
  other 
  change, 
  physical 
  or 
  

   chemical, 
  seemed 
  to 
  have 
  taken 
  place. 
  The 
  biotite 
  dust, 
  however, 
  

   which 
  had 
  formed 
  in 
  the 
  carbonic-acid 
  water 
  showed 
  some 
  impor- 
  

   tant 
  changes. 
  The 
  minute 
  spangles 
  had 
  all 
  become 
  thoroughly 
  

   hydrated 
  as 
  in 
  the 
  above 
  case 
  ; 
  but 
  they 
  had, 
  besides, 
  become 
  consi- 
  

   derably 
  lighter 
  in 
  colour. 
  Some 
  of 
  the 
  more 
  highly 
  decolorized 
  flakes, 
  

   when 
  examined 
  by 
  polarized 
  light, 
  now 
  exhibited 
  practically 
  no 
  pleo- 
  

   chroism, 
  whereas 
  the 
  very 
  finest 
  plates 
  detached 
  from 
  the 
  portion 
  

   of 
  the 
  mineral 
  which 
  had 
  not 
  been 
  exposed 
  to 
  the 
  action 
  of 
  either 
  of 
  

   the 
  waters 
  were 
  distinctly 
  dichroic, 
  as 
  also 
  were 
  those 
  which 
  had 
  

   lain 
  in 
  the 
  pure 
  distilled 
  water 
  only 
  ; 
  so 
  that 
  the 
  result 
  of 
  the 
  pro- 
  

   longed 
  action 
  of 
  the 
  carbonated 
  water 
  on 
  the 
  minute 
  biotite 
  flakes 
  

   detached 
  from 
  the 
  suspended 
  mass 
  had 
  been 
  to 
  convert 
  them 
  into 
  

   ordinary 
  hydromuscovites. 
  Superficially, 
  the 
  suspended 
  biotite 
  masses 
  

   were 
  found, 
  like 
  the 
  minute 
  flakes, 
  to 
  have 
  lessened 
  in 
  degree 
  of 
  

   lustre 
  and 
  slightly 
  in 
  hardness, 
  to 
  have 
  lost 
  considerably 
  in 
  tough- 
  

   ness 
  and 
  elasticity, 
  and 
  to 
  have 
  acquired 
  a 
  more 
  or 
  less 
  unctuous 
  

   feel. 
  The 
  solid 
  fragment 
  of 
  biotite 
  which 
  had 
  been 
  placed 
  in 
  the 
  

   carbonated 
  water 
  likewise 
  showed 
  slight 
  decoloration 
  or 
  bleaching 
  

   on 
  the 
  edges 
  of 
  the 
  superficial 
  laminae. 
  The 
  mass 
  which 
  had 
  hung 
  

   in 
  the 
  pure 
  water 
  showed 
  no 
  such 
  change. 
  An 
  examination 
  was 
  

   next 
  made 
  of 
  the 
  liquids 
  to 
  the 
  action 
  of 
  which 
  the 
  biotites 
  had 
  

   been 
  subjected. 
  Both 
  were 
  separately 
  and 
  very 
  carefully 
  filtered 
  

   from 
  their 
  solid 
  contents 
  and 
  evaporated 
  to 
  dryness 
  in 
  separate 
  

   dishes. 
  The 
  pure 
  distilled 
  water 
  passed 
  off 
  without 
  leaving 
  any 
  

   residue, 
  showing, 
  as 
  in 
  the 
  case 
  of 
  the 
  muscovites, 
  that 
  the 
  only 
  

   chemical 
  change 
  resulting 
  from 
  its 
  action 
  on 
  the 
  biotite 
  was 
  that 
  of 
  

   hydration. 
  

  

  Q. 
  J.G. 
  S. 
  No. 
  178. 
  2d 
  

  

  