﻿

  

  H 
  3 
  

  

  Prehnite 
  

  

  H, 
  

  

  

  

  / 
  

  

  Si0 
  4 
  

  

  = 
  A1 
  2 
  ^ 
  

  

  :SiO, 
  

  

  \ 
  

  

  Al 
  

  

  

  

  SiO, 
  

  

  4 
  

  

  = 
  Ca 
  s 
  = 
  

  

  :SiO, 
  

  

  — 
  Al 
  

  

  

  \ 
  

  

  SiO, 
  

  

  II 
  

  

  Na 
  2 
  

  

  - 
  Ca 
  - 
  

  

  Si0 
  4 
  

  

  II 
  

  

  Na, 
  

  

  / 
  

  

  

  

  Soda 
  sarcolite 
  

  

  

  F. 
  W. 
  Clarke— 
  Constitution 
  of 
  Melilite 
  and 
  Gehlenite. 
  477 
  

  

  been 
  mentioned 
  are 
  also 
  orthosilicates, 
  and 
  the 
  true 
  components 
  

   of 
  melilite 
  and 
  gehlenite 
  ought 
  in 
  all 
  probability 
  to 
  follow 
  the 
  

   same 
  rule. 
  The 
  tetragonal 
  sarcolite, 
  for 
  example, 
  has 
  the 
  same 
  

   molecular 
  ratios 
  as 
  the 
  isometric 
  garnet, 
  and 
  a 
  probable 
  struc- 
  

   tural 
  difference 
  between 
  the 
  two 
  species 
  is 
  easy 
  to 
  represent 
  

   graphically. 
  Including 
  an 
  analogous 
  compound, 
  prehnite, 
  the 
  

   following 
  constitutional 
  formulae 
  are 
  at 
  least 
  probable: 
  

  

  / 
  Si0 
  4 
  = 
  Al, 
  = 
  Si0 
  4 
  \ 
  / 
  Si0 
  4 
  = 
  Al, 
  = 
  Si0 
  4 
  \ 
  

  

  Al 
  — 
  Si0 
  4 
  = 
  Ca 
  3 
  = 
  Si0 
  4 
  — 
  Al 
  Al 
  — 
  Si0 
  4 
  = 
  Ca 
  3 
  ee 
  Si0 
  4 
  — 
  Al 
  

  

  \ 
  Si0 
  4 
  = 
  Ca 
  3 
  = 
  Si0 
  4 
  / 
  \ 
  Si0 
  4 
  - 
  Ca 
  - 
  Si0 
  4 
  / 
  

  

  Garnet 
  

  

  /Si0 
  4 
  = 
  Al 
  2 
  =Si0 
  4 
  \ 
  

  

  Al 
  — 
  Si0 
  4 
  = 
  Ca 
  3 
  = 
  Si0 
  4 
  — 
  Al 
  

  

  \ 
  Si0 
  4 
  - 
  Ca 
  - 
  Si0 
  4 
  / 
  

  

  II 
  II 
  

  

  Ca 
  Ca 
  

  

  Sarcolite 
  

  

  Natural 
  sarcolite 
  is 
  a 
  mixture 
  of 
  the 
  lime 
  salt 
  and 
  the 
  soda 
  

   salt, 
  but 
  the 
  replacement 
  of 
  lime 
  by 
  soda 
  is 
  not 
  necessarily 
  

   complete. 
  The 
  formulation 
  given 
  here 
  fits 
  best 
  into 
  the 
  gen- 
  

   eral 
  scheme, 
  which 
  will 
  be 
  developed 
  presently. 
  The 
  two 
  

   sarcolites, 
  however, 
  as 
  Schaller 
  has 
  shown, 
  are 
  probable 
  com- 
  

   ponents 
  of 
  melilite 
  and 
  gehlenite. 
  That 
  is 
  an 
  important 
  step 
  

   forward. 
  

  

  On 
  comparing 
  the 
  trustworthy 
  analyses 
  of 
  melilite 
  and 
  geh- 
  

   lenite 
  two 
  striking 
  differences 
  appear. 
  In 
  melilite 
  the 
  ratio 
  

   of 
  oxygen 
  to 
  silicon 
  is 
  slightly 
  lower 
  than 
  that 
  required 
  for 
  the 
  

   orthosilicate 
  radicle 
  Si0 
  4 
  ; 
  and 
  for 
  that 
  reason 
  trisilicate 
  mole- 
  

   cules 
  may 
  be 
  assumed 
  as 
  possibly 
  present. 
  This 
  assumption 
  is 
  

   justified 
  by 
  familiar 
  analogies 
  as 
  in 
  the 
  isomorphism 
  of 
  albite 
  

   and 
  anorthite, 
  and 
  of 
  the 
  scapolitic 
  minerals 
  marialite 
  and 
  

   meionite. 
  In 
  each 
  of 
  these 
  pairs 
  a 
  trisilicate 
  and 
  an 
  orthosili- 
  

   cate 
  are 
  morphologically 
  equivalent. 
  In 
  gehlenite, 
  on 
  the 
  

   other 
  hand, 
  the 
  oxygen 
  is 
  largely 
  in 
  excess 
  of 
  the 
  orthosilicate, 
  

   and 
  basic 
  molecules 
  must 
  be 
  taken 
  into 
  account. 
  This 
  con- 
  

   dition 
  is 
  satisfied 
  by 
  assuming 
  the 
  presence 
  in 
  gehlenite 
  of 
  the 
  

  

  univalent 
  group 
  — 
  Al 
  ^o^ 
  ^ 
  a 
  5 
  which 
  in 
  Schaller's 
  system 
  is 
  

  

  characteristic 
  of 
  his 
  basic 
  silicate, 
  u 
  velardenite." 
  In 
  the 
  pres- 
  

   ent 
  discussion 
  its 
  function 
  will 
  be 
  differently 
  shown. 
  Further- 
  

   more, 
  natural 
  melilite 
  always 
  contains 
  soda, 
  presumably 
  as 
  soda 
  

  

  