﻿Pratt 
  — 
  Separation 
  of 
  Alumina 
  from 
  Molten 
  Magmas. 
  229 
  

  

  onsly 
  mentioned. 
  The 
  greater 
  part 
  of 
  the 
  alumina 
  separates 
  

   out 
  as 
  corundum. 
  

  

  5. 
  In 
  a 
  number 
  of 
  these 
  peridotite 
  formations 
  feldspar 
  has 
  

   been 
  found 
  which 
  is 
  undoubtedly 
  one 
  of 
  the 
  original 
  minerals 
  

   of 
  the 
  rock 
  and 
  not 
  a 
  secondary 
  product. 
  

  

  At 
  the 
  Cullakeenee 
  Mine, 
  Buck 
  Creek, 
  Clay 
  County, 
  there 
  

   is, 
  at 
  one 
  of 
  the 
  contact 
  corundum 
  veins, 
  a 
  large 
  amount 
  of 
  

   feldspar 
  and 
  hornblende 
  which 
  have 
  separated 
  out 
  with 
  the 
  

   corundum 
  : 
  again 
  at 
  the 
  Bad 
  Creek 
  Mine, 
  Sapphire, 
  Jackson 
  

   County, 
  the 
  corundum 
  is 
  found 
  associated 
  with 
  feldspar 
  and 
  

   biotite 
  mica. 
  In 
  both 
  places 
  they 
  are 
  lime-soda 
  feldspars, 
  and 
  

   this 
  occurrence 
  of 
  feldspar 
  and 
  corundum 
  in 
  a 
  peridotite 
  rock 
  

   indicates 
  that 
  the 
  molten 
  magma 
  contained 
  some 
  of 
  the 
  alkali 
  

   and 
  alkali 
  earth 
  oxides, 
  as 
  Na^O, 
  K^O 
  and 
  CaO, 
  and 
  that 
  a 
  

   portion 
  of 
  the 
  alumina 
  united 
  in 
  the 
  formation 
  of 
  the 
  feldspar 
  

   molecule 
  and 
  that 
  the 
  rest 
  of 
  it 
  separated 
  out 
  as 
  corundum. 
  

   In 
  a 
  magma 
  of 
  this 
  type, 
  there 
  is 
  present 
  a 
  large 
  amount 
  of 
  

   magnesia 
  which 
  forms 
  the 
  raagnesian 
  silicates 
  but 
  apparently 
  

   has 
  no 
  tendency 
  to 
  unite 
  with 
  the 
  alumina 
  to 
  form 
  the 
  spinel 
  

   molecule 
  ; 
  but 
  on 
  the 
  other 
  hand 
  the 
  small 
  amount 
  of 
  the 
  

   alkalies 
  and 
  alkali-earth 
  oxides 
  do 
  unite 
  with 
  a 
  definite 
  amount 
  

   of 
  the 
  alumina 
  to 
  form 
  the 
  feldspar 
  molecule. 
  This 
  again 
  

   would 
  seem 
  to 
  indicate, 
  that 
  in 
  the 
  presence 
  of 
  a 
  certain 
  

   amount 
  of 
  the 
  alkali 
  and 
  alkali-earth 
  oxides 
  and 
  silica 
  that 
  if 
  

   there 
  is 
  enough 
  silica 
  present 
  to 
  unite 
  with 
  these 
  oxides 
  and 
  

   the 
  magnesia, 
  there 
  is 
  no 
  tendency 
  for 
  the 
  magnesia 
  to 
  unite 
  

   with 
  the 
  alumina 
  to 
  form 
  any 
  tj^pe 
  of 
  magnesia-alumina 
  min- 
  

   erals. 
  

  

  Morozewicz"^ 
  has 
  recently 
  described 
  the 
  formation 
  of 
  corun- 
  

   dum 
  from 
  a 
  number 
  of 
  magmas 
  that 
  were 
  supersaturated 
  with 
  

   alumina. 
  The 
  supersaturated 
  alumina-silicate 
  magmas 
  had 
  the 
  

   general 
  composition 
  MeO 
  . 
  mAl^Og 
  . 
  nSi02(Me 
  = 
  K^^ 
  ^N'a^, 
  Ca, 
  and 
  

   ?z 
  = 
  2 
  — 
  13). 
  In 
  these 
  magmas, 
  on 
  cooling, 
  all 
  the 
  excess 
  of 
  

   alumina 
  (over 
  m 
  = 
  1) 
  separates 
  out 
  in 
  the 
  form 
  of 
  corundum 
  

   crystals, 
  this 
  taking 
  place 
  when 
  magnesia 
  and 
  iron 
  were 
  not 
  

   present. 
  

  

  In 
  these 
  experiments, 
  the 
  silicates 
  that 
  were 
  used 
  to 
  form 
  

   the 
  magmas 
  were 
  for 
  the 
  most 
  part 
  those 
  of 
  calcium, 
  sodium 
  

   and 
  potassium. 
  When 
  alumina 
  was 
  present 
  in 
  them 
  in 
  excess 
  

   of 
  911 
  = 
  1, 
  it 
  would 
  separate 
  out 
  in 
  the 
  form 
  of 
  corundum 
  ; 
  

   but 
  when 
  the 
  magma 
  was 
  not 
  saturated 
  with 
  alumina 
  or 
  m 
  was 
  

   equal 
  to 
  1 
  or 
  less, 
  all 
  the 
  alumina 
  was 
  used 
  up 
  in 
  the 
  formation 
  

   of 
  double 
  silicates. 
  When 
  magnesia 
  was 
  present, 
  the 
  excess 
  of 
  

   alumina 
  united 
  with 
  the 
  magnesia 
  to 
  form 
  the 
  mineral 
  spinel, 
  

   and 
  if 
  not 
  enough 
  magnesia 
  was 
  present 
  to 
  unite 
  with 
  all 
  of 
  

  

  * 
  Tschermak's 
  Min. 
  u. 
  Petrogr, 
  Mittlieilungen, 
  xviii, 
  pp. 
  1-90 
  and 
  105-240, 
  

   1898. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  YIIT, 
  No. 
  45. 
  — 
  September, 
  1899. 
  

   16 
  

  

  