﻿476 
  F. 
  W. 
  Clarke 
  — 
  Constitution 
  of 
  Melilite 
  and 
  Gefilenite. 
  

  

  Art. 
  XLV. 
  — 
  The 
  Constitution 
  of 
  Melilite 
  and 
  Gehlenite;* 
  

   by 
  Frank 
  Wigglesworth 
  Clarke. 
  

  

  Although 
  many 
  attempts 
  have 
  been 
  made 
  to 
  determine 
  the 
  

   chemical 
  constitution 
  of 
  melilite 
  and 
  gehlenite, 
  no 
  one 
  of 
  them 
  

   can 
  be 
  accepted 
  as 
  absolutely 
  conclusive. 
  The 
  analyses 
  of 
  the 
  

   two 
  minerals 
  vary 
  widely, 
  showing 
  that 
  they 
  are 
  not 
  true 
  

   species 
  but 
  crystalline 
  mixtures 
  analogous 
  to 
  those 
  found 
  in 
  the 
  

   feldspar 
  series, 
  the 
  micas, 
  the 
  scapolites, 
  and 
  the 
  garnet 
  group. 
  

   The 
  problem 
  is, 
  to 
  determine 
  the 
  individual 
  components 
  of 
  

   melilite 
  and 
  gehlenite, 
  and 
  in 
  such 
  a 
  way 
  as 
  to 
  show 
  their 
  rela- 
  

   tions 
  to 
  each 
  other, 
  and 
  to 
  all 
  minerals 
  of 
  similar 
  chemical 
  com- 
  

   position 
  or 
  type. 
  

  

  The 
  most 
  recent, 
  and 
  decidedly 
  the 
  most 
  successful 
  attempt 
  

   to 
  interpret 
  the 
  constitution 
  of 
  these 
  minerals 
  is 
  due 
  to 
  W. 
  T. 
  

   Schaller.f 
  He 
  regards 
  melilite 
  and 
  gehlenite 
  as 
  mixtures 
  of 
  

   four 
  definite 
  silicates, 
  namely, 
  akermanite, 
  Mg 
  4 
  Ca 
  8 
  Si 
  9 
  O 
  30 
  ; 
  sarco- 
  

   lite, 
  Al 
  2 
  Ca 
  3 
  Si 
  3 
  12 
  ; 
  soda 
  sarcolite, 
  Al 
  2 
  Na 
  6 
  Si 
  3 
  12 
  , 
  and 
  " 
  velarde- 
  

   nite,'' 
  Al 
  2 
  Ca 
  2 
  Si0 
  7 
  . 
  The 
  last 
  of 
  these 
  silicates 
  is 
  well 
  known 
  as 
  

   an 
  artificial 
  compound, 
  but 
  not 
  definitely 
  as 
  a 
  natural 
  mineral. 
  

   All 
  four 
  silicates 
  are 
  tetragonal, 
  and 
  Schaller 
  has 
  shown 
  that 
  

   their 
  formulae 
  can 
  be 
  adjusted 
  to 
  the 
  analyses 
  of 
  melilite 
  and 
  

   gehlenite 
  with 
  a 
  remarkable 
  degree 
  of 
  accuracy. 
  The 
  crystal- 
  

   lographic 
  and 
  chemical 
  data 
  are 
  in 
  harmony, 
  but 
  unfortunately, 
  

   they 
  do 
  not 
  cover 
  the 
  whole 
  ground. 
  The 
  relations 
  of 
  melilite 
  

   and 
  gehlenite 
  to 
  other 
  minerals, 
  such 
  as 
  anorthite, 
  garnet, 
  zoi- 
  

   site, 
  vesuvianite, 
  and 
  meionite, 
  are 
  not 
  indicated 
  by 
  Schaller's 
  

   method 
  of 
  formulation, 
  and 
  so 
  the 
  fundamental 
  problem 
  is 
  

   incompletely 
  solved. 
  A 
  great 
  advance 
  has 
  been 
  made, 
  but 
  

   much 
  still 
  remains 
  to 
  be 
  done. 
  The 
  present 
  communication 
  

   represents 
  an 
  effort 
  to 
  find 
  a 
  more 
  general 
  interpretation 
  of 
  the 
  

   lime-alumina 
  silicates, 
  not 
  only 
  with 
  regard 
  to 
  their 
  chemical 
  

   composition, 
  but 
  also 
  with 
  reference 
  to 
  their 
  associations 
  in 
  

   nature, 
  their 
  mode 
  of 
  origin, 
  and 
  their 
  alterations. 
  

  

  To 
  Schaller's 
  formulation 
  of 
  melilite 
  and 
  gehlenite 
  there 
  are 
  

   obvious 
  objections, 
  which, 
  however, 
  he 
  has 
  shown 
  to 
  be 
  not 
  

   absolutely 
  insuperable. 
  The 
  four 
  primary 
  compounds 
  of 
  his 
  

   scheme 
  can 
  be 
  represented 
  by 
  a 
  single 
  type 
  of 
  formula, 
  but 
  to 
  

   that 
  procedure 
  there 
  are 
  alternatives 
  which 
  seem 
  to 
  be 
  more 
  

   reasonable. 
  The 
  compounds 
  are 
  not 
  alike 
  chemically; 
  for 
  

   akermanite 
  is 
  non-aluminous, 
  "velardenite 
  " 
  is 
  extremely 
  basic, 
  

   and 
  sarcolite 
  is 
  most 
  simply 
  formulated 
  as 
  a 
  normal 
  orthosili- 
  

   cate. 
  The 
  other 
  lime-alumina 
  silicates 
  which 
  have 
  already 
  

  

  * 
  Published 
  by 
  permission 
  of 
  the 
  Director 
  of 
  the 
  U. 
  S. 
  Geological 
  Survey. 
  

   fU. 
  S. 
  Geol. 
  Survey, 
  Bull. 
  610, 
  pp. 
  106-126. 
  Schaller 
  gives 
  a 
  good 
  sum- 
  

   mary 
  of 
  earlier 
  interpretations, 
  which 
  need 
  not 
  be 
  repeated 
  here. 
  

  

  