﻿112 
  F. 
  W. 
  Clarhe 
  — 
  Constitution 
  of 
  Tourmaline. 
  

  

  The 
  two 
  analyses 
  by 
  Penfield 
  and 
  Foote, 
  however, 
  conform 
  

   sharply 
  to 
  their 
  formula, 
  thus 
  : 
  

  

  De 
  Kalb, 
  white 
  H,g.,^B3.^3Si,0 
  

  

  Haddam 
  Neck, 
  green 
  H,'3.,3B3.„^Si,03^.,, 
  

  

  31-48 
  

  

  The 
  Gouverneur 
  and 
  Hamburgh 
  tourmalines 
  represent 
  the 
  

   extreme 
  range 
  of 
  variation 
  ; 
  a 
  variation 
  which 
  is 
  too 
  large 
  to 
  

   be 
  safely 
  set 
  aside 
  as 
  due 
  to 
  analytical 
  errors 
  or 
  to 
  impurities 
  

   in 
  the 
  material 
  analyzed. 
  Some 
  of 
  the 
  formulae 
  approximate 
  

   to 
  mine, 
  some 
  to 
  that 
  of 
  Penfield 
  and 
  Foote, 
  and 
  hence 
  it 
  

   seems 
  probable 
  that 
  neither 
  formula, 
  without 
  some 
  qualifica- 
  

   tion, 
  can 
  safely 
  be 
  taken 
  as 
  final. 
  

  

  In 
  order 
  to 
  be 
  satisfactory, 
  a 
  constitutional 
  formula 
  must 
  

   fulfil 
  several 
  conditions. 
  First, 
  it 
  must 
  adequately 
  express 
  the 
  

   composition 
  of 
  the 
  compound 
  in 
  question, 
  covering 
  all 
  of 
  its 
  

   variations. 
  Secondly, 
  it 
  must 
  be 
  readily 
  applicable 
  to 
  the 
  full 
  

   discussion 
  of 
  analyses, 
  so 
  that 
  the 
  different 
  isomorphous 
  salts 
  

   which 
  are 
  commingled 
  in 
  a 
  mineral 
  species 
  can 
  be 
  separately 
  

   identified 
  and 
  given 
  reasonable 
  expressions. 
  Finally, 
  it 
  should 
  

   indicate 
  the 
  relations 
  between 
  a 
  species 
  and 
  the 
  other 
  minerals 
  

   with 
  which 
  it 
  is 
  allied, 
  or 
  into 
  which 
  it 
  commonly 
  alters. 
  A 
  

   formula 
  can 
  be 
  fully 
  adopted 
  only 
  when 
  all 
  of 
  these 
  conditions 
  

   are 
  satisfied. 
  The 
  third 
  condition, 
  which 
  relates 
  to 
  function, 
  

   is 
  equally 
  important 
  with 
  the 
  other 
  two. 
  

  

  With 
  the 
  tourmalines, 
  the 
  micas 
  seem 
  to 
  be 
  most 
  nearly 
  akin. 
  

   In 
  each 
  group 
  we 
  have 
  to 
  consider 
  comminglings 
  of 
  isomorphous 
  

   molecules, 
  and 
  when 
  tourmaline 
  alters, 
  a 
  mica 
  is 
  commonly 
  

   the 
  product 
  of 
  the 
  reaction. 
  In 
  composition, 
  also, 
  the 
  two 
  

   groups 
  show 
  an 
  apparent 
  parallelism. 
  With 
  the 
  lithia 
  mica, 
  

   lepidolite, 
  lithia 
  tourmalines 
  occur 
  ; 
  with 
  muscovite 
  and 
  biotite, 
  

   the 
  common 
  iron 
  tourmaline 
  is 
  associated 
  ; 
  and 
  the 
  magnesian 
  

   tourmalines, 
  which 
  show 
  the 
  minimum 
  of 
  alumina 
  in 
  their 
  

   composition, 
  are 
  similarly 
  allied 
  to 
  phlogopite. 
  This 
  relation- 
  

   ship, 
  if 
  it 
  is 
  real, 
  should 
  be 
  suggested 
  in 
  the 
  formulae 
  assigned 
  

   to 
  the 
  several 
  species. 
  

  

  To 
  the 
  commoner 
  micas 
  a 
  simple 
  series 
  of 
  formulae 
  can 
  be 
  

   easily 
  given, 
  thus 
  : 
  

  

  Muscovite... 
  Al3(SiOj3KH,, 
  

  

  Biotite. 
  - 
  AK(SiO 
  j3Mg;ivH, 
  

  

  Phlogopite 
  A];(SiOj3Mg3KII.,; 
  

  

  and 
  to 
  these 
  types 
  or 
  mixtures 
  of 
  them, 
  most 
  micas 
  are 
  refer- 
  

   rable. 
  The 
  variations 
  and 
  exceptions 
  have 
  been 
  considered 
  

   elsewhere, 
  and 
  need 
  not 
  be 
  discussed 
  here. 
  

  

  With 
  these 
  fundamental 
  molecules 
  the 
  corresponding 
  salts 
  

   of 
  the 
  tourmaline 
  acid 
  H^gBgSieOg,, 
  or 
  Pl„Al,BgSi„03, 
  are 
  struc- 
  

  

  