﻿180 
  Foote 
  and 
  Bradley 
  — 
  Solid 
  Solution 
  in 
  Minerals. 
  

  

  Art. 
  XXII. 
  — 
  On 
  Solid 
  Solution 
  in 
  Minerals. 
  I 
  V. 
  ' 
  The 
  

   Composition 
  of 
  Amorphous 
  Minerals 
  as 
  Illustrated 
  by 
  

   Chrysocolla 
  ; 
  by 
  H. 
  W. 
  Foote 
  and 
  W. 
  M. 
  Bradley. 
  

  

  At 
  the 
  present 
  time 
  a 
  definite 
  chemical 
  formula 
  is 
  ascribed 
  

   to 
  nearly 
  every 
  well-known 
  mineral. 
  The 
  variations 
  in 
  com- 
  

   position 
  which 
  actually 
  occur 
  can 
  in 
  most 
  cases 
  be 
  satisfactorily 
  

   explained 
  by 
  assuming 
  isomorphous 
  replacement. 
  Usually 
  

   this 
  consists 
  in 
  one 
  or 
  more 
  metals 
  being 
  partly 
  substituted 
  for 
  

   the 
  metal 
  in 
  the 
  ideal 
  compound, 
  or, 
  what 
  amounts 
  to 
  the 
  same 
  

   thing, 
  a 
  molecule 
  of 
  the 
  ideal 
  compound 
  is 
  substituted 
  by 
  

   a 
  molecule 
  of 
  another 
  similar 
  in 
  type. 
  This 
  common 
  case 
  

   occurs 
  when 
  potash 
  replaces 
  soda 
  in 
  albite. 
  In 
  rarer 
  cases, 
  

   compounds 
  or 
  radicals 
  of 
  different 
  type 
  appear 
  to 
  be 
  isomor- 
  

   phous, 
  or 
  capable 
  of 
  forming 
  solid 
  solutions. 
  For 
  instance, 
  in 
  

   the 
  plagioclases 
  anorthite 
  replaces 
  albite, 
  in 
  nephelite 
  there 
  is 
  

   a 
  variable 
  excess 
  of 
  silica,* 
  and 
  in 
  pyrrhotite 
  an 
  excess 
  of 
  

   sulphur.f 
  

  

  The 
  minerals 
  which 
  appear 
  to 
  present 
  the 
  greatest 
  difficulty 
  

   in 
  the 
  relation 
  between 
  actual 
  composition 
  and 
  formula 
  are 
  

   those 
  which 
  commonly 
  occur 
  in 
  the 
  amorphous 
  condition, 
  but 
  

   in 
  these 
  cases, 
  as 
  with 
  well-crystallized 
  minerals, 
  definite 
  for- 
  

   mulae 
  are 
  commonly 
  given 
  in 
  all 
  reference 
  works 
  on 
  the 
  subject. 
  

   An 
  examination 
  of 
  the 
  facts 
  will 
  show, 
  however, 
  that 
  in 
  many 
  

   amorphous 
  minerals 
  the 
  actual 
  composition 
  found 
  may 
  differ 
  

   very 
  widely 
  indeed 
  from 
  the 
  theoretical 
  value 
  required 
  by 
  the 
  

   formula. 
  

  

  As 
  an 
  illustration 
  of 
  this 
  type 
  of 
  mineral, 
  we 
  have 
  chosen 
  

   chrysocolla, 
  to 
  which 
  the 
  formula 
  CuSi0 
  3 
  . 
  2H 
  2 
  is 
  commonly 
  

   assigned. 
  We 
  give 
  below 
  the 
  ratios 
  for 
  Si0 
  2 
  . 
  CuO 
  and 
  H 
  2 
  

   calculated 
  for 
  this 
  mineral 
  from 
  the 
  analyses 
  given 
  in 
  Dana's 
  

   Mineralogy 
  and 
  Hintze's 
  Handbuch. 
  The 
  numbers 
  are 
  those 
  

   given 
  in 
  the 
  two 
  reference 
  books 
  mentioned. 
  We 
  have 
  omitted 
  

   from 
  Hintze's 
  list 
  the 
  ratios 
  of 
  analyses 
  as 
  given 
  by 
  both 
  Dana 
  

   and 
  Hintze. 
  

  

  In 
  considering 
  these 
  ratios 
  the 
  fact 
  must 
  be 
  taken 
  into 
  account 
  

   that 
  some 
  of 
  the 
  analyses 
  are 
  undoubtedly 
  inaccurate 
  and 
  the 
  

   material 
  used 
  was 
  impure. 
  Too 
  much 
  weight, 
  therefore, 
  cannot 
  

   be 
  laid 
  on 
  the 
  above 
  results. 
  Taking 
  the 
  results 
  as 
  they 
  are, 
  

   however, 
  only 
  nine 
  of 
  the 
  thirty-one 
  silica 
  ratios 
  show 
  satis- 
  

   factory 
  values 
  between 
  0'90 
  and 
  1*10. 
  Six 
  ratios 
  are 
  under 
  *90 
  

   and 
  sixteen 
  above 
  1 
  -10. 
  Of 
  the 
  ratios 
  for 
  water, 
  three 
  only 
  

   are 
  between 
  1*80 
  and 
  2*20, 
  eight 
  are 
  under 
  1*80 
  and 
  twenty 
  

  

  *This 
  Journal, 
  xxxi, 
  25, 
  1911. 
  

  

  f 
  Allen, 
  Crenshaw 
  and 
  Johnson, 
  ibid., 
  xxxiii, 
  169, 
  1912. 
  

  

  