﻿228 
  Pratt 
  — 
  Separation 
  of 
  Alumina 
  from 
  Molten 
  Magmas. 
  

  

  4. 
  Peridotite 
  rocks 
  containing 
  chroraite 
  and 
  corundum, 
  but 
  

   no 
  spinel. 
  

  

  5. 
  Peridotite 
  rocks 
  containing 
  corundum 
  and 
  feldspar. 
  

  

  1 
  and 
  2. 
  In 
  the 
  analyses 
  of 
  the 
  chromites 
  that 
  have 
  come 
  

   under 
  the 
  author's 
  notice, 
  all 
  but 
  two 
  show 
  the 
  presence 
  of 
  

   alumina 
  and 
  magnesia 
  in 
  varying 
  amounts. 
  In 
  some 
  of 
  them 
  

   the 
  percentage 
  of 
  alumina 
  and 
  magnesia 
  is 
  very 
  large 
  and 
  

   where 
  in 
  the 
  ordinary 
  chromite 
  the 
  formula 
  may 
  be 
  represented 
  

   by 
  9 
  (reO.Cr,03), 
  (MgO.CrA), 
  2 
  (MgO.Al 
  A) 
  ; 
  at 
  one 
  local- 
  

   ity 
  near 
  Webster. 
  Jackson 
  County, 
  a 
  chromite 
  {raitchellite)^ 
  

   was 
  found 
  which 
  had 
  the 
  formula 
  (FeO.Cr^Og), 
  (MgO.Cr.Og), 
  

   2 
  (MgO.Al 
  2O3). 
  Thus 
  when 
  there 
  is 
  considerable 
  chromite 
  

   found 
  in 
  these 
  rocks, 
  there 
  is 
  usually 
  no, 
  or 
  at 
  most 
  but 
  a 
  trace 
  

   of 
  corundum 
  present, 
  and 
  from 
  what 
  has 
  been 
  said 
  above 
  

   regarding 
  the 
  analyses 
  of 
  chromite, 
  it 
  would 
  seem 
  to 
  be 
  the 
  

   case 
  that 
  when 
  there 
  is 
  but 
  little 
  alumina 
  and 
  only 
  a 
  small 
  

   excess 
  of 
  magnesia 
  in 
  peridotite 
  magmas, 
  containing 
  consider- 
  

   able 
  chromic 
  oxide, 
  that 
  they 
  unite 
  to 
  form 
  the 
  spinel 
  molecule 
  

   which 
  instead 
  of 
  separating 
  out 
  independently 
  as 
  the 
  mineral 
  

   spinel, 
  enters 
  into 
  combination 
  with 
  the 
  FeO-Cr^Og 
  molecule 
  

   in 
  the 
  mineral 
  chromite. 
  

  

  3. 
  At 
  a 
  few 
  of 
  the 
  peridotite 
  localities 
  both 
  corundum 
  and 
  

   spinel 
  have 
  been 
  found, 
  as 
  at 
  the 
  Corundum 
  Hill 
  Mine, 
  where 
  

   a 
  iine-grained, 
  almost 
  black 
  spinel 
  occurs 
  through 
  which 
  is 
  

   scattered 
  particles 
  and 
  masses 
  of 
  corundum 
  ; 
  and 
  at 
  the 
  Carter 
  

   Mine 
  near 
  Democrat, 
  where 
  there 
  is 
  a 
  great 
  abundance 
  of 
  cor- 
  

   undum 
  and 
  a 
  green 
  black 
  spinel. 
  In 
  peridotite 
  magmas 
  of 
  

   this 
  type, 
  it 
  would 
  seem 
  that 
  there 
  has 
  been 
  an 
  excess 
  of 
  mag- 
  

   nesia 
  present 
  which 
  has 
  united 
  with 
  a 
  portion 
  of 
  the 
  alumina 
  

   present 
  to 
  form 
  the 
  molecule 
  MgO.Al203, 
  which 
  separated 
  

   out 
  as 
  spinel. 
  The 
  remaining 
  portion 
  of 
  the 
  alumina 
  formed 
  

   corundum. 
  

  

  Undoubtedly 
  there 
  is 
  a 
  strong 
  affinity 
  between 
  magnesia 
  and 
  

   alumina 
  which 
  tends 
  to 
  form 
  the 
  spinel 
  molecule 
  ; 
  but 
  in 
  these 
  

   peridotite 
  magmas, 
  it 
  is 
  only 
  the 
  excess 
  of 
  magnesia 
  over 
  that 
  

   required 
  for 
  the 
  normal 
  magnesian 
  silicates 
  that 
  has 
  united 
  

   with 
  the 
  alumina. 
  Except 
  alteration 
  products 
  there 
  are 
  no 
  

   alumina-magnesia 
  silicates 
  found 
  in 
  these 
  rocks, 
  showing 
  that 
  

   in 
  magmas 
  of 
  this 
  type 
  there 
  is 
  no 
  tendency 
  for 
  the 
  alumina 
  

   to 
  unite 
  with 
  the 
  magnesia 
  in 
  forming 
  double 
  silicates. 
  

  

  4. 
  At 
  most 
  of 
  the 
  peridotite 
  localities 
  there 
  has 
  been 
  no 
  

   spinel 
  at 
  all 
  observed, 
  but 
  there 
  is 
  often 
  a 
  considerable 
  quan- 
  

   tity 
  of 
  corundum 
  found. 
  The 
  chromite 
  at 
  all 
  these 
  localities 
  

   shows 
  the 
  presence 
  of 
  a 
  certain 
  percentage 
  of 
  alumina 
  and 
  

   magnesia. 
  Thus 
  the 
  small 
  excess 
  of 
  magnesia 
  unites 
  with 
  a 
  

   definite 
  amount 
  of 
  the 
  alumina, 
  but 
  instead 
  of 
  separating 
  out 
  

   as 
  spinel, 
  it 
  separates 
  out 
  in 
  the 
  chromite 
  molecule 
  as 
  previ- 
  

  

  