﻿138 
  ON 
  THE 
  CRYSTALS 
  DEVELOPED 
  IN 
  VERMICULITE 
  BY 
  HEAT. 
  

  

  my 
  request 
  by 
  professor 
  Bache, 
  yielded 
  much 
  alumina, 
  some 
  lime, 
  

   lithia 
  and 
  oxide 
  of 
  iron, 
  but 
  neither 
  magnesia 
  nor 
  potash. 
  It 
  is 
  possible 
  

   that 
  the 
  vermiculite, 
  which 
  bears 
  the 
  appearance 
  of 
  a 
  decomposed 
  

   mineral, 
  has 
  lost 
  part 
  of 
  its 
  potash 
  and 
  silex, 
  like 
  the 
  porcelain 
  earth 
  ; 
  

   and 
  that 
  the 
  remaining 
  part 
  combines 
  under 
  the 
  influence 
  of 
  heat 
  in 
  

   proper 
  proportion 
  to 
  form 
  the 
  plates, 
  while 
  a 
  portion 
  containing 
  no 
  

   potash 
  remains 
  interposed 
  between 
  them. 
  

  

  These 
  crystals 
  then 
  are 
  probably 
  a 
  potash 
  and 
  lithia 
  mica, 
  of 
  which 
  

   the 
  crystalline 
  form 
  indicates 
  two 
  axes 
  of 
  refraction, 
  and 
  the 
  constitu- 
  

   ents 
  of 
  which, 
  being 
  contained 
  in 
  the 
  vermiculite, 
  are 
  combined 
  and 
  

   crystallize 
  suddenly 
  by 
  the 
  action 
  of 
  heat 
  ; 
  the 
  reverse 
  operation 
  of 
  

   crystallizing 
  by 
  cooling. 
  

  

  The 
  formation 
  of 
  mica 
  in 
  the 
  minerals 
  of 
  Mount 
  Vesuvius 
  may 
  be 
  

   explained 
  on 
  similar 
  principles, 
  though 
  in 
  that 
  case 
  we 
  have 
  a 
  mag- 
  

   nesian 
  mica, 
  with 
  a 
  single 
  axis 
  of 
  refraction. 
  

  

  