﻿Foote 
  and 
  Bradley 
  — 
  Constant 
  Composition 
  of 
  Albite. 
  47 
  

  

  Art. 
  YI. 
  — 
  On 
  Solid 
  Solution 
  in 
  Minerals. 
  III. 
  The 
  

   Constant 
  Composition 
  of 
  Albite 
  ; 
  by 
  H. 
  W. 
  Foote 
  and 
  

   W. 
  M. 
  Bradley. 
  

  

  In 
  recent 
  years, 
  the 
  fact 
  is 
  becoming 
  more 
  and 
  more 
  

   recognized 
  that 
  certain 
  minerals 
  are 
  capable 
  of 
  taking 
  up 
  or 
  

   dissolving, 
  in 
  solid 
  solution, 
  foreign 
  components 
  which 
  from 
  

   their 
  nature 
  can 
  hardly 
  be 
  considered 
  as 
  isomorphous 
  mixtures 
  

   in 
  the 
  ordinary 
  sense. 
  Thus, 
  the 
  mineral 
  pyrrhotite 
  has 
  

   recently 
  been 
  shown* 
  to 
  be 
  a 
  solid 
  solution 
  of 
  sulphur 
  in 
  fer- 
  

   rous 
  sulphide 
  ; 
  nephelite 
  always 
  contains 
  an 
  excess 
  of 
  silica,f 
  

   and 
  analcite, 
  of 
  a 
  hydrated 
  silica.;}; 
  It 
  is 
  impossible 
  in 
  most 
  

   cases, 
  with 
  our 
  present 
  knowledge, 
  to 
  state 
  with 
  certainty 
  the 
  

   form 
  in 
  which 
  the 
  admixture 
  occurs. 
  The 
  dissolved 
  sulphur 
  

   of 
  pyrrhotite, 
  for 
  instance, 
  may 
  be 
  as 
  pyrite, 
  the 
  excess 
  silica 
  

   of 
  nephelite 
  may 
  be 
  as, 
  albite 
  and 
  the 
  excess 
  of 
  hydrated 
  silica 
  

   in 
  analcite 
  as 
  some 
  other 
  silicate, 
  but 
  in 
  whatever 
  form 
  the 
  

   admixture 
  occurs, 
  the 
  solid 
  solution 
  is 
  quite 
  different 
  in 
  charac- 
  

   ter 
  from 
  ordinary 
  isomorphous 
  replacement, 
  such 
  as 
  is 
  found 
  

   in 
  the 
  feldspars 
  where 
  potash 
  replaces 
  soda. 
  

  

  Whether 
  this 
  capacity 
  to 
  form 
  solid 
  solutions 
  with 
  com- 
  

   pounds 
  of 
  different 
  type 
  is 
  fairly 
  common 
  among 
  silicates 
  or 
  is 
  

   limited 
  to 
  a 
  few 
  minerals 
  can 
  hardly 
  be 
  stated 
  with 
  certainty 
  

   at 
  present. 
  This 
  is 
  chiefly 
  due 
  to 
  the 
  fact 
  that 
  most 
  minerals 
  

   have 
  been 
  chosen 
  for 
  analysis 
  on 
  account 
  of 
  their 
  purity 
  and 
  

   freedom 
  from 
  mechanical 
  admixture 
  with 
  other 
  substances. 
  

   If, 
  however, 
  a 
  compound 
  can 
  dissolve 
  another, 
  it 
  is 
  evident 
  

   that 
  it 
  will 
  have 
  taken 
  up 
  the 
  maximum 
  amount 
  when 
  it 
  is 
  

   found 
  mixed 
  mechanically 
  with 
  an 
  excess 
  of 
  the 
  substance 
  dis- 
  

   solved. 
  Thus, 
  nephelite 
  occurs 
  with 
  varying 
  amounts 
  of 
  excess 
  

   silica 
  in 
  solid 
  solution, 
  but 
  it 
  contains 
  the 
  maximum 
  amount 
  

   when 
  it 
  crystallizes 
  with 
  albite, 
  which 
  contains 
  more 
  silica 
  than 
  

   nephelite 
  but 
  the 
  same 
  proportions 
  of 
  soda 
  and 
  alumina. 
  In 
  

   order, 
  then, 
  to 
  determine 
  whether 
  a 
  mineral 
  can 
  form 
  a 
  solid 
  

   solution 
  with 
  another, 
  and 
  to 
  what 
  extent 
  this 
  is 
  possible, 
  the 
  

   composition 
  of 
  the 
  mineral 
  when 
  associated 
  with 
  the 
  other 
  

   must 
  be 
  determined. 
  If 
  the 
  composition 
  remains 
  fixed, 
  inde- 
  

   pendent 
  of 
  association, 
  no 
  solid 
  solution 
  takes 
  place. 
  If 
  it 
  

   changes 
  with 
  its 
  association, 
  then 
  the 
  maximum 
  amount 
  of 
  

   solid 
  solution 
  can 
  be 
  determined. 
  § 
  

  

  Albite 
  sometimes 
  occurs, 
  associated 
  with 
  an 
  excess 
  of 
  its 
  

  

  * 
  Allen, 
  Crenshaw 
  and 
  Johnston, 
  this 
  Journal, 
  xxxiii, 
  169, 
  1912. 
  

  

  t 
  Ibid., 
  xxxi, 
  25, 
  1911. 
  % 
  Ibid., 
  xxxiii, 
  433, 
  1912. 
  

  

  § 
  This 
  leaves 
  out 
  of 
  account 
  the 
  possible 
  effect 
  of 
  temperature 
  on 
  the 
  

   maximum 
  amount 
  dissolved, 
  but 
  this 
  is 
  probably 
  small 
  under 
  actual 
  condi- 
  

   tions 
  of 
  crystallization. 
  

  

  