﻿480 
  F. 
  W. 
  Clarke 
  — 
  Constitution 
  of 
  Melilite 
  and 
  Gehlenite. 
  

   Calculated 
  analyses 
  of 
  melilite. 
  

  

  Si0 
  

  

  41-21 
  

  

  40-64 
  

  

  39-16 
  

  

  40-99 
  

  

  40-97 
  

  

  40-15 
  

  

  43-36 
  

  

  A1,0, 
  

  

  14-13 
  

  

  13-36 
  

  

  15-26 
  

  

  13-04 
  

  

  12-93 
  

  

  15-11 
  

  

  10-96 
  

  

  CaO 
  

  

  39-93 
  

  

  43*00 
  

  

  42-36 
  

  

  42-50 
  

  

  42-88 
  

  

  41-56 
  

  

  42-32 
  

  

  Na 
  a 
  O 
  

  

  4-73 
  

  

  3-00 
  

  

  3-22 
  

  

  3-47 
  

  

  3-22 
  

  

  3-18 
  

  

  3-36 
  

  

  10000 
  100*00 
  100-00 
  100-00 
  100-00 
  100-00 
  100-00 
  

  

  The 
  agreement 
  between 
  the 
  reduced 
  and 
  calculated 
  analyses 
  

   is 
  very 
  close. 
  In 
  many 
  cases 
  the 
  normal 
  melilite 
  silicate 
  

   Al 
  2 
  (Si0 
  4 
  ) 
  6 
  Ca 
  9 
  predominates, 
  forming 
  from 
  60 
  to 
  70 
  per 
  cent 
  of 
  

   the 
  mixed 
  crystals. 
  The 
  two 
  sarcolites 
  come 
  next, 
  with 
  the 
  

   hypothetical 
  trisilicate 
  molecule 
  in 
  subordinate 
  amount. 
  The 
  

   existence 
  of 
  the 
  trisilicate, 
  however, 
  is 
  subject 
  to 
  some 
  doubt. 
  

   Melilite 
  is 
  prone 
  to 
  contain 
  impurities, 
  inclusions 
  of 
  other 
  min- 
  

   erals, 
  and 
  the 
  excess 
  of 
  silica 
  shown 
  by 
  the 
  analyses 
  may 
  per- 
  

   haps 
  be 
  due 
  to 
  their 
  presence. 
  The 
  melilite 
  of 
  Capo 
  di 
  Bove, 
  

   for 
  example, 
  commonly 
  contains 
  inclusions 
  of 
  leucite,* 
  and 
  it 
  

   is 
  possible 
  that 
  dissolved 
  silica 
  may 
  sometime 
  be 
  present 
  also. 
  

   The 
  largest 
  proportion 
  of 
  trisilicate 
  appears 
  in 
  the 
  massive 
  

   melilite 
  from 
  Colorado, 
  which 
  is 
  more 
  likely 
  to 
  contain 
  impuri- 
  

   ties 
  than 
  the 
  crystallized 
  mineral. 
  If 
  the 
  excess 
  of 
  silica 
  is 
  

   really 
  dissolved 
  silica, 
  then 
  the 
  trisilicate 
  molecule 
  would 
  be 
  

   replaced 
  by 
  an 
  equivalent 
  amount 
  of 
  soda 
  sarcolite, 
  and 
  the 
  

   formulse 
  of 
  natural 
  melilite 
  would 
  be 
  simplified. 
  It 
  would 
  

   then 
  be 
  represented 
  as 
  a 
  crystalline 
  mixture 
  of 
  the 
  two 
  sarco- 
  

   lites 
  with 
  the 
  compound 
  Al 
  2 
  (Si0 
  4 
  ) 
  6 
  Ca 
  9 
  in 
  varying 
  proportions. 
  

  

  It 
  has 
  already 
  been 
  pointed 
  out 
  that 
  gehlenite 
  differs 
  from 
  

   melilite 
  in 
  its 
  ratio 
  of 
  silicon 
  to 
  oxygen 
  ; 
  and 
  that 
  the 
  difference 
  

   may 
  be 
  accounted 
  for 
  by 
  assuming 
  the 
  presence 
  in 
  it 
  of 
  the 
  

  

  univalent 
  groups 
  — 
  Al 
  <^ 
  \ 
  Ca. 
  That 
  is, 
  gehlenite 
  is 
  a 
  basic 
  

  

  silicate, 
  and 
  the 
  basic 
  group 
  may 
  be 
  combined 
  in 
  one 
  of 
  several 
  

   ways. 
  The 
  compound 
  containing 
  it 
  may 
  have 
  the 
  formula 
  

   Al(Si0 
  4 
  ) 
  3 
  (A10 
  2 
  Ca) 
  9 
  , 
  or, 
  more 
  probably, 
  a 
  formula 
  derived 
  from 
  

   that 
  of 
  the 
  melilite 
  silicate, 
  thus 
  : 
  

  

  / 
  Si0 
  4 
  = 
  Ca 
  3 
  = 
  Si0 
  4 
  \ 
  '. 
  

   Al 
  — 
  Si0 
  4 
  == 
  Ca, 
  = 
  Si0 
  4 
  — 
  Al 
  Al 
  

  

  \ 
  Si(X 
  EECa 
  =SiO; 
  / 
  

  

  / 
  Si0 
  4 
  = 
  

  

  Ca, 
  

  

  = 
  SiO, 
  

  

  \ 
  

  

  — 
  SiO, 
  = 
  

  

  Ca„ 
  

  

  = 
  Si0 
  4 
  

  

  — 
  Al 
  

  

  \ 
  SiO, 
  

  

  

  SiO, 
  

  

  / 
  

  

  (A10,Ca) 
  s 
  

  

  

  (AIO.C 
  

  

  *). 
  

  

  Gehle 
  

  

  nite 
  

  

  silicate 
  

  

  

  Melilite 
  silicate 
  

  

  *See 
  Stelzner, 
  Nenes 
  Jahrb., 
  Beil. 
  Bd. 
  ii, 
  369, 
  1883. 
  

  

  