﻿certain 
  Micas, 
  Vermiculites 
  and 
  Chlorites. 
  251 
  

  

  fore 
  assume 
  that 
  mineral 
  to 
  be 
  present 
  as 
  an 
  impurity. 
  Deduct- 
  

   ing 
  from 
  the 
  molecular 
  ratios 
  given 
  above 
  the 
  quantity 
  of 
  talc 
  

   indicated 
  by 
  experiment 
  we 
  get 
  for 
  the 
  empirical 
  formula 
  of 
  

   the 
  mineral 
  the 
  expression 
  

  

  Al 
  M 
  K 
  M 
  Mg 
  M 
  H 
  11T 
  . 
  Si0 
  4 
  ),„0 
  28 
  „ 
  

  

  which 
  becomes, 
  if 
  the 
  excess 
  of 
  oxygen 
  is 
  regarded 
  as 
  hydroxyl, 
  

   with 
  (MgOH) 
  109 
  as 
  observed, 
  

  

  Mg„ 
  9 
  K 
  S0 
  H 
  88 
  ,(MgOH) 
  10 
  ,(AlHAUOH), 
  

  

  The 
  small 
  excess 
  of 
  hydroxyl 
  is 
  probably 
  due 
  to 
  undistributed 
  

   errors 
  of 
  analysis, 
  and 
  may 
  be 
  added 
  to 
  the 
  MgOH, 
  bringing 
  

   the 
  latter 
  to 
  118, 
  and 
  reducing 
  the 
  Mg 
  to 
  820. 
  Then, 
  general- 
  

   izing, 
  by 
  uniting 
  all 
  the 
  univalent 
  groups 
  and 
  atoms 
  we 
  get 
  as 
  

   an 
  ultimate 
  formula 
  

  

  Mg 
  e20 
  (SiO 
  4 
  )„ 
  5 
  R' 
  I13 
  „ 
  

   which 
  equals, 
  almost 
  exactly, 
  

  

  125 
  Mg 
  3 
  (Si0 
  4 
  ) 
  2 
  R' 
  2 
  + 
  223 
  Mg,(Si0 
  4 
  ) 
  9 
  R' 
  4 
  , 
  

  

  a 
  result 
  in 
  accordance 
  with 
  our 
  serpentine-chlorite 
  theory. 
  

   The 
  distribution 
  of 
  the 
  several 
  components 
  of 
  R' 
  is, 
  however, 
  

   not 
  clear, 
  and 
  remains 
  to 
  be 
  ascertained. 
  ~No 
  other 
  discussion 
  

   of 
  the 
  analysis 
  appears 
  to 
  give 
  as 
  satisfactory 
  results 
  as 
  this, 
  

   and 
  we 
  have 
  tried 
  several 
  methods 
  of 
  reduction, 
  representing 
  

   various 
  hypotheses. 
  

  

  One 
  other 
  mineral 
  examined 
  during 
  this 
  investigation 
  remains 
  

   to 
  be 
  noticed 
  ; 
  a 
  pale 
  yellowish-green 
  mica 
  collected 
  by 
  Mr. 
  G. 
  

   P. 
  Merrill 
  at 
  a 
  granite 
  quarry 
  in 
  Auburn, 
  Me., 
  near 
  where 
  the 
  

   Maine 
  Central 
  railroad 
  crosses 
  the 
  Androscoggin 
  river. 
  It 
  

   occurs 
  in 
  direct 
  contact 
  with 
  ordinary, 
  broadly 
  foliated 
  mus- 
  

   covite, 
  sometimes 
  forming 
  marginal 
  growths 
  about 
  the 
  plates 
  

   of 
  the 
  latter 
  mineral, 
  like 
  lepidolite. 
  Analysis 
  gave 
  : 
  

  

  Si0 
  2 
  46-54 
  

  

  A1 
  2 
  3 
  34-96 
  

  

  Fe 
  2 
  3 
  1-59 
  

  

  MgO 
  0-32 
  

  

  K 
  2 
  10-38 
  

  

  Na 
  2 
  0-41 
  

  

  F 
  ._. 
  __ 
  none 
  

  

  H 
  2 
  0, 
  105° 
  0-71 
  

  

  " 
  ignition 
  4*72 
  

  

  99-63 
  

  

  This 
  is 
  the 
  composition 
  of 
  muscovite, 
  which 
  the 
  mica 
  undoubt- 
  

   edly 
  is. 
  The 
  case 
  is 
  interesting, 
  however, 
  as 
  showing 
  a 
  

   secondary 
  growth 
  of 
  muscovite 
  on 
  muscovite, 
  with 
  a 
  marked 
  

   difference 
  in 
  outward 
  appearance 
  between 
  the 
  two 
  formations. 
  

  

  Laboratory 
  U. 
  S. 
  Geological 
  Survey, 
  Washington, 
  D. 
  C, 
  April 
  27, 
  1891. 
  

  

  