﻿506 
  Lane 
  and 
  Sharjpless 
  — 
  Notes 
  on 
  Michigan 
  Minerals. 
  

  

  never 
  more 
  than 
  faintly 
  pleochroic. 
  The 
  angle 
  from 
  c 
  : 
  # 
  is 
  

   between 
  15° 
  and 
  20°. 
  We 
  endeavored 
  to 
  get 
  the 
  curve 
  of 
  the 
  

   extinction 
  angles 
  in 
  the 
  prismatic 
  zone, 
  by 
  revolving 
  a 
  fiber 
  

   in 
  Cachet's 
  "cuve 
  goniometrique" 
  filled 
  with 
  mono-brom- 
  

   naphthalin. 
  Owing 
  to 
  the 
  twinning 
  the 
  results 
  were 
  not 
  satis- 
  

   factory 
  but 
  in 
  a 
  general 
  way 
  the 
  curve 
  starting 
  from 
  a 
  0° 
  

   extinction 
  at 
  010 
  rose 
  with 
  an 
  initial 
  slope 
  of 
  about 
  7 
  until 
  

   perpendicular 
  to 
  the 
  cleavage 
  faces 
  the 
  extinction 
  was 
  nearly 
  

   20°. 
  After 
  that 
  it 
  varied 
  but 
  little. 
  

  

  From 
  these 
  observations'* 
  and 
  from 
  the 
  relative 
  retardations 
  

   of 
  pinacoidal 
  sections 
  as 
  manifested 
  in 
  their 
  polarization, 
  values 
  

   of 
  — 
  2 
  Y 
  between 
  50° 
  and 
  80° 
  are 
  deduced. 
  The 
  specific 
  gravity 
  

   of 
  a 
  specimen 
  contaminated 
  with 
  quartz 
  alone 
  was 
  found 
  to 
  be 
  

   3*2 
  to 
  3-3. 
  

  

  § 
  3. 
  The 
  purest 
  material 
  at 
  hand 
  in 
  sufficient 
  bulk 
  for 
  analy- 
  

   sis 
  seemed 
  to 
  be 
  some 
  from 
  the 
  Champion 
  Mine, 
  after 
  remov- 
  

   ing 
  all 
  the 
  iron 
  oxides 
  from 
  it. 
  Unfortunately 
  the 
  crushing 
  

   of 
  a 
  larger 
  amount 
  for 
  analysis 
  introduced 
  unexpected 
  impuri- 
  

   ties. 
  The 
  mineral 
  is 
  changed 
  to 
  talc 
  in 
  spots, 
  and 
  there 
  was 
  

   some 
  entangled 
  quartz 
  though 
  apparently 
  not 
  enough 
  to 
  

   account 
  for 
  the 
  high 
  percentage 
  of 
  SiO 
  a 
  . 
  

  

  The 
  following 
  analysis 
  which 
  was 
  made 
  on 
  material 
  first 
  

   treated 
  with 
  the 
  magnet 
  to 
  remove 
  magnetite, 
  and 
  then 
  briefly 
  

   with 
  HC1 
  to 
  remove 
  martite 
  and 
  hematite, 
  has 
  therefore 
  merely 
  

   a 
  qualitative 
  value, 
  but 
  shows 
  very 
  clearly 
  by 
  the 
  absence 
  of 
  

   lime 
  that 
  the 
  mineral 
  is 
  not 
  actinolite. 
  

  

  Si0 
  2 
  ._ 
  76-32 
  

  

  Al 
  a 
  O, 
  .-. 
  0-56 
  

  

  Fe 
  2 
  3 
  _ 
  0-99 
  

  

  FeO 
  6-96 
  

  

  MgO 
  12-47 
  

  

  CaO 
  '___. 
  ---. 
  

  

  (Na,K) 
  s 
  O 
  tr. 
  

  

  H 
  2 
  2-80 
  

  

  Sum 
  100-20 
  

  

  The 
  alteration 
  to 
  talcose 
  matter 
  is 
  strictly 
  pseudomorphic 
  and 
  

   follows 
  lines 
  of 
  alteration 
  parallel 
  to 
  the 
  polysynthetic 
  twin- 
  

   ning 
  (100). 
  It 
  must 
  be 
  quite 
  extensive. 
  The 
  mineral 
  is 
  in 
  

   small 
  fibers, 
  associated 
  with 
  quartz 
  and 
  iron 
  ores, 
  commonly 
  

  

  *It 
  may 
  be 
  shown, 
  starting 
  from 
  Levy's 
  equation 
  3, 
  p. 
  20, 
  that 
  cos 
  5 
  V 
  (sin 
  %+ 
  1) 
  

   = 
  cos 
  2 
  0(sin 
  x— 
  1) 
  — 
  lj 
  when 
  V 
  is 
  half 
  the 
  optical 
  augle, 
  0= 
  £ 
  £ 
  : 
  a, 
  and 
  x 
  the 
  

   angle 
  from 
  the 
  plane 
  (010) 
  to 
  the 
  prismatic 
  plane 
  in 
  which 
  the 
  angle 
  of 
  extinction 
  

   first 
  becomes 
  equal 
  to 
  6. 
  We 
  may 
  also 
  show 
  that 
  cos' 
  2 
  V 
  (tan 
  + 
  cot 
  6)= 
  tan 
  6 
  + 
  

  

  (initial 
  steepness 
  of 
  curve 
  of 
  extinctions, 
  i. 
  e., 
  — 
  — 
  \ 
  

  

  dxoj 
  

  

  