﻿230 
  Pratt 
  — 
  Sejparation 
  of 
  A 
  himin 
  a 
  from 
  Molten 
  Magmas. 
  

  

  the 
  alumina, 
  some 
  corundum 
  was 
  formed. 
  When 
  there 
  was 
  an 
  

   excess 
  of 
  silica, 
  or 
  n 
  is 
  greater 
  than 
  6, 
  the 
  excess 
  of 
  alumina 
  

   unites 
  with 
  the 
  silica 
  to 
  form 
  the 
  mineral 
  sillimanite 
  and 
  the 
  

   over 
  excess 
  of 
  alumina 
  would 
  separate 
  out 
  as 
  corundum. 
  

   When, 
  however, 
  magnesia 
  and 
  iron 
  are 
  present 
  with 
  the 
  excess 
  

   of 
  silica 
  and 
  alumina, 
  cordierite 
  and 
  spinel 
  are 
  the 
  minerals 
  

   formed. 
  

  

  From 
  these 
  experiments 
  it 
  would 
  seem 
  that 
  where 
  a 
  magma 
  

   is 
  composed 
  of 
  silicates 
  of 
  the 
  alkali 
  and 
  alkali-earth 
  metals, 
  

   sodium, 
  potassium 
  and 
  calcium, 
  that 
  the 
  alumina 
  dissolved 
  in 
  

   this 
  magma 
  will 
  to 
  a 
  certain 
  point 
  unite 
  to 
  form 
  double 
  silicates 
  

   of 
  alumina 
  with 
  these 
  other 
  bases 
  ; 
  but 
  when 
  the 
  ratio 
  of 
  

   alumina 
  to 
  these 
  bases 
  is 
  greater 
  than 
  1, 
  the 
  excess 
  of 
  alumina 
  

   will 
  separate 
  out 
  as 
  corundum 
  except 
  when 
  influenced 
  as 
  just 
  

   described. 
  Magnesia, 
  it 
  would 
  seem, 
  does 
  not 
  have 
  as 
  strong 
  an 
  

   aflinity 
  for 
  the 
  formation 
  of 
  silicates, 
  and 
  when 
  present 
  in 
  the 
  

   magmas, 
  influences 
  the 
  separation 
  of 
  the 
  alumina 
  as 
  a 
  double 
  

   salt 
  of 
  magnesia 
  and 
  alumina 
  in 
  the 
  formation 
  of 
  the 
  mineral 
  

   spinel, 
  or 
  with 
  an 
  excess 
  of 
  both 
  alumina 
  and 
  silica 
  in 
  the 
  

   formation 
  of 
  the 
  mineral 
  cordierite. 
  

  

  In 
  some 
  earlier 
  experiments, 
  Morozewicz"^ 
  has 
  shown 
  that 
  

   alumina 
  will 
  readily 
  dissolve 
  in 
  a 
  molten 
  magma 
  having 
  a 
  com- 
  

   position 
  approximately 
  that 
  of 
  the 
  basic 
  magnesian 
  rocks 
  ; 
  and 
  

   that 
  upon 
  cooling 
  the 
  alumina 
  separates 
  out 
  as 
  corundum 
  and 
  

   spinel. 
  

  

  There 
  seems 
  to 
  be 
  but 
  little 
  tendency 
  for 
  the 
  alumina 
  to 
  

   unite 
  with 
  the 
  magnesia 
  to 
  form 
  the 
  double 
  silicates 
  or 
  with 
  

   the 
  magnesia 
  alo.ne 
  to 
  form 
  the 
  mineral 
  spinel, 
  MgO.AlgOg, 
  

   except 
  when 
  there 
  is 
  an 
  excess 
  of 
  MgO, 
  when 
  some 
  spinel 
  is 
  

   formed, 
  but 
  the 
  greater 
  portion 
  of 
  the 
  alumina 
  separates 
  as 
  

   corundum. 
  This 
  would 
  seem 
  to 
  show 
  that 
  the 
  aflinity 
  of 
  mag- 
  

   nesia 
  for 
  silica 
  in 
  the 
  formation 
  of 
  a 
  silicate 
  molecule 
  is 
  

   stronger 
  than 
  for 
  alumina 
  to 
  form 
  the 
  aluminate 
  molecule. 
  

  

  Siunmary. 
  — 
  From 
  what 
  has 
  been 
  observed 
  in 
  nature 
  and 
  

   from 
  the 
  experiments 
  that 
  have 
  been 
  made 
  in 
  the 
  laboratory, 
  

   it 
  seems 
  that 
  the 
  separation 
  of 
  alumina 
  as 
  corundum 
  from 
  

   molten 
  magmas 
  is 
  dependent 
  upon 
  the 
  composition 
  of 
  the 
  

   chemical 
  compounds 
  that 
  are 
  the 
  basis 
  of 
  the 
  magma, 
  upon 
  the 
  

   oxides 
  that 
  are 
  dissolved 
  with 
  the 
  alumina 
  in 
  the 
  magma 
  and 
  

   upon 
  the 
  amount 
  of 
  alumina 
  itself. 
  

  

  1. 
  When 
  the 
  magma 
  is 
  a 
  calcium-sodium-potassium 
  silicate, 
  

   no 
  alumina 
  held 
  in 
  solution 
  by 
  such 
  a 
  magma 
  will 
  separate 
  out 
  

   as 
  corundum 
  except 
  when 
  the 
  ratio 
  of 
  the 
  alumina 
  to 
  the 
  other 
  

   bases 
  is 
  more 
  than 
  1 
  : 
  1 
  and 
  the 
  ratio 
  of 
  the 
  silica 
  is 
  less 
  than 
  6. 
  

  

  * 
  Zeitschr. 
  fiir 
  Kryst., 
  vol. 
  xxiv, 
  p. 
  281, 
  1895. 
  

  

  