﻿•3992 
  

  

  3432 
  

  

  Lane 
  and 
  Keller 
  — 
  Notes 
  on 
  Michigan 
  Minerals. 
  503 
  

  

  Mol. 
  ratio. 
  

  

  SiO,.. 
  24-29 
  -4048 
  

  

  TiO„ 
  0-28 
  -0035 
  

  

  Al 
  a 
  O 
  a 
  34-00 
  -3333 
  j 
  

  

  Fe",0 
  3 
  10-55 
  -0659 
  1 
  

  

  FeO 
  20-51 
  -2850^1 
  

  

  MnO 
  trace 
  1 
  

  

  M^O 
  1-29 
  -0430 
  f 
  

  

  CaO 
  061 
  -0152J 
  

  

  K 
  2 
  0-97 
  -0103 
  

  

  Ka 
  2 
  0-35 
  -0059>-3906 
  

  

  H 
  2 
  6 
  75 
  -3744 
  

  

  Sura 
  99-60 
  

  

  From 
  these 
  figures 
  we 
  deduce 
  the 
  formula 
  H 
  16 
  Fe.Al 
  lfi 
  Si 
  8 
  S6 
  

   or 
  8H 
  a 
  0-7FeO 
  ■ 
  8Al 
  a 
  O 
  a 
  ■ 
  8Si0 
  3 
  , 
  which 
  is 
  nearly 
  identical 
  with 
  

   that 
  now 
  generally 
  accepted 
  for 
  sismondine.* 
  It 
  will 
  be 
  ob- 
  

   served, 
  however, 
  that 
  the 
  composition 
  of 
  our 
  mineral, 
  as 
  we 
  

   haye 
  determined 
  it, 
  differs 
  from 
  the 
  published 
  analyses 
  of 
  the 
  

   latter 
  in 
  two 
  respects 
  : 
  the 
  iron 
  is 
  largely 
  in 
  the 
  ferric 
  condi- 
  

   tion, 
  and 
  alkalies 
  occur 
  in 
  notable 
  quantity. 
  Since 
  the- 
  micro- 
  

   scopic 
  examination 
  of 
  the 
  material 
  revealed 
  only 
  traces 
  of 
  

   sericite, 
  and 
  the 
  magnetite 
  had 
  been 
  carefully 
  extracted 
  with 
  

   the 
  magnet, 
  it 
  is 
  evident 
  that 
  the 
  ferric 
  oxide 
  as 
  well 
  as 
  the 
  

   alkalies 
  are 
  essential 
  constituents 
  of 
  this 
  chloritoid. 
  The 
  ferric 
  

   oxide 
  without 
  doubt 
  replaces 
  part 
  of 
  the 
  alumina, 
  while 
  the 
  

   alkalies, 
  it 
  may 
  be 
  assumed, 
  are 
  substitutes 
  for 
  some 
  of 
  the 
  

   kydroxyl-water. 
  We 
  have 
  reason 
  to 
  believe 
  that 
  alkalies 
  have 
  

   been 
  overlooked 
  in 
  many 
  of 
  the 
  former 
  analyses 
  of 
  chloritoid 
  

   and 
  the 
  allied 
  species. 
  An 
  examination 
  of 
  the 
  masonite 
  from 
  

   Natick, 
  for 
  instance, 
  showed 
  them 
  to 
  be 
  present 
  to 
  upwards 
  of 
  

   two 
  per 
  cent 
  — 
  the 
  soda 
  predominating 
  — 
  and 
  a 
  qualitative 
  test 
  

   disclosed 
  small 
  amounts 
  of 
  both 
  potash 
  and 
  soda 
  in 
  the 
  Pre- 
  

   gratten 
  occurrence. 
  The 
  titanic 
  acid 
  in 
  our 
  analysis 
  was 
  doubt- 
  

   less 
  contributed 
  by 
  a 
  slight 
  admixture 
  of 
  ilmenite 
  or 
  rutile. 
  

  

  § 
  5. 
  Xow, 
  comparing 
  our 
  data 
  with 
  those 
  generally 
  given, 
  

   we 
  find 
  that 
  all 
  authorities 
  agree, 
  — 
  first, 
  in 
  the 
  pleochroism 
  ; 
  

   secondly, 
  that 
  y 
  is 
  the 
  positive 
  acute 
  bisectrix 
  ; 
  thirdly, 
  that 
  

   there 
  is 
  a 
  marked 
  dispersion, 
  p<y. 
  Lacroix 
  also 
  mentions 
  the 
  

   horizontal 
  dispersion. 
  There 
  is 
  a 
  decided 
  difference, 
  however, 
  

   as 
  to 
  the 
  direction 
  of 
  the 
  axes 
  of 
  elasticity, 
  y, 
  ,3 
  and 
  a. 
  It 
  is 
  

   perhaps 
  worth 
  noting 
  that 
  in 
  Rosenbusch's 
  " 
  Microscopic 
  

   Physiography 
  " 
  there 
  is 
  a 
  statement 
  on 
  page 
  494, 
  that 
  in 
  

   masonite 
  the 
  pleochroism 
  of 
  a 
  is 
  blue 
  and 
  ft 
  green. 
  This 
  is 
  

  

  * 
  Groth, 
  Tabell. 
  Uebersicht, 
  3 
  ed., 
  p. 
  13 
  8. 
  

  

  Am. 
  Jour. 
  Sci— 
  Third 
  Series, 
  Vol. 
  XLII, 
  No. 
  252.— 
  December, 
  1391. 
  

   34 
  

  

  