﻿Pratt— 
  Separation 
  of 
  Alumina 
  from 
  Molten 
  Magmas. 
  231 
  

  

  2. 
  If 
  magnesia 
  and 
  iron 
  are 
  present 
  in 
  the 
  above 
  magma, 
  

   corundum 
  will 
  not 
  form 
  unless 
  there 
  is 
  more 
  than 
  enough 
  

   alumina 
  to 
  unite 
  with 
  the 
  magnesia 
  and 
  iron. 
  

  

  3. 
  When 
  the 
  magma 
  is 
  composed 
  of 
  a 
  magnesium 
  silicate 
  

   without 
  excess 
  of 
  magnesia, 
  all 
  the 
  alumina 
  held 
  by 
  such 
  a 
  

   magma 
  will 
  separate 
  out 
  as 
  corundum. 
  

  

  4. 
  Where 
  there 
  is 
  an 
  excess 
  of 
  magnesia 
  in 
  the 
  magma 
  just 
  

   described, 
  this 
  will 
  unite 
  with 
  a 
  portion 
  of 
  the 
  alumina 
  to 
  form 
  

   spinel 
  and 
  the 
  rest 
  of 
  the 
  alumina 
  will 
  separate 
  out 
  as 
  corun- 
  

   dum. 
  

  

  5. 
  Where 
  there 
  is 
  chromic 
  oxide 
  present 
  in 
  a 
  magma 
  com- 
  

   posed 
  essentially 
  of. 
  a 
  magnesium 
  silicate 
  (as 
  the 
  peridotite 
  

   rocks) 
  and 
  only 
  a 
  very 
  little 
  alumina 
  and 
  magnesia 
  are 
  present, 
  

   these, 
  uniting, 
  separate 
  out 
  with 
  chromic 
  oxide 
  to 
  form 
  the 
  

   mineral 
  chromite 
  and 
  no 
  corundum 
  or 
  spinel 
  are 
  formed. 
  

  

  6. 
  When 
  peridotite 
  magmas 
  contain, 
  besides 
  the 
  alumina, 
  

   oxides 
  of 
  the 
  alkalies 
  and 
  alkali-earths, 
  as 
  soda, 
  potash 
  and 
  lime, 
  

   a 
  portion 
  of 
  the 
  alumina 
  is 
  used 
  in 
  uniting 
  with 
  these 
  oxides 
  

   and 
  silica 
  to 
  form 
  feldspar. 
  

  

  7. 
  There 
  is 
  a 
  strong 
  tendency 
  for 
  the 
  alumJna 
  to 
  unite 
  with 
  

   the 
  alkali 
  and 
  alkali-earth 
  oxides 
  to 
  form 
  double 
  silicates 
  like 
  

   feldspars, 
  whether 
  such 
  silicates 
  form 
  the 
  chief 
  minerals 
  of 
  the 
  

   resulting 
  rock 
  or 
  are 
  present 
  only 
  in 
  relatively 
  small 
  amount. 
  

   There 
  is, 
  however, 
  but 
  little 
  tendency 
  for 
  the 
  alumina 
  to 
  unite 
  

   with 
  magnesia 
  to 
  form 
  double 
  silicates 
  when 
  the 
  magma 
  is 
  a 
  

   magnesium 
  silicate. 
  

  

  North 
  Carolina 
  Geolog-ical 
  Survey, 
  June, 
  1899. 
  

  

  