﻿F. 
  W. 
  Clarice 
  — 
  Constitution 
  of 
  Melilite 
  and 
  Gehlenite. 
  483 
  

  

  Epidote 
  and 
  its 
  congeners, 
  piedmontite, 
  allanite 
  and 
  han- 
  

   cockite, 
  are 
  of 
  course 
  analogous 
  to 
  zoisite. 
  As 
  for 
  meionite, 
  

   the 
  carbon 
  ate-meionite 
  and 
  sulphate-meionite 
  of 
  JBorgstrom* 
  

   contain 
  the 
  atomic 
  groups 
  -Ca-C0 
  3 
  -Ca- 
  and 
  -Ca-S0 
  4 
  -Ca- 
  

   replacing 
  the 
  groups 
  -Ca-O-Ca- 
  of 
  the 
  formula 
  given 
  here. 
  

   Possibly 
  a 
  similar 
  group 
  -Ca-Si0 
  3 
  -Ca- 
  may 
  account 
  for 
  the 
  

   excesses 
  of 
  silica 
  shown 
  in 
  some 
  analyses 
  of 
  meionite, 
  which 
  

   are 
  not 
  always 
  ascribable 
  to 
  marialite. 
  

  

  We 
  now 
  see 
  that 
  melilite 
  and 
  gehlenite 
  fall 
  into 
  place 
  as 
  

   members 
  of 
  a 
  well-defined 
  group 
  of 
  silicates 
  of 
  similar 
  chemi- 
  

   cal 
  constitution. 
  Furthermore, 
  they 
  frequently 
  occur 
  in 
  nature 
  

   as 
  products 
  of 
  contact 
  metamorphism 
  in 
  limestones. 
  That 
  is, 
  

   they 
  have 
  a 
  common 
  origin. 
  Garnet, 
  epidote, 
  vesuvianite, 
  

   and 
  scapolites 
  are 
  well-known 
  as 
  associates 
  in 
  such 
  rocks, 
  and 
  

   anorthite 
  is 
  sometimes 
  found 
  with 
  them. 
  Sarcolite 
  and 
  also 
  

   vesuvianite 
  are 
  found 
  in 
  the 
  ejected 
  limestone 
  blocks 
  of 
  Monte 
  

   Somma 
  ; 
  but 
  prehnite, 
  presumably 
  a 
  derivative 
  of 
  garnet, 
  is 
  

   exceptional. 
  I 
  do 
  not 
  find 
  it 
  recorded 
  as 
  an 
  associate 
  of 
  the 
  

   other 
  species. 
  

  

  At 
  Orawitza 
  gehlenite 
  is 
  found 
  in 
  rolled 
  pebbles 
  containing 
  

   grains 
  of 
  vesuvianite 
  ; 
  and 
  the 
  Colorado 
  melilite 
  is 
  associated 
  

   with 
  both 
  garnet 
  and 
  vesuvianite, 
  which 
  are 
  products 
  of 
  its 
  

   alteration. 
  The 
  garnet 
  is 
  probably 
  derived 
  from 
  its 
  isomer, 
  

   sarcolite; 
  and 
  the 
  vesuvianite 
  appears 
  to 
  be 
  a 
  basic 
  derivative 
  

   of 
  the 
  melilite 
  silicate, 
  Al 
  2 
  (Si0 
  4 
  ) 
  G 
  Ca 
  9 
  . 
  

  

  Anorthite 
  alters 
  into 
  scapolite 
  ; 
  vesuvianite 
  and 
  gehlenite 
  to 
  

   garnet 
  ; 
  and 
  garnet 
  to 
  epidote 
  and 
  scapolite. 
  All 
  of 
  these 
  min- 
  

   erals 
  alter 
  into 
  micas, 
  and 
  the 
  magnesian 
  varieties 
  into 
  chlorites 
  

   also. 
  These 
  facts 
  all 
  strengthen 
  the 
  argument 
  that 
  the 
  several 
  

   silicates 
  which 
  are 
  considered 
  here 
  are 
  intimately 
  related 
  both 
  

   structurally 
  and 
  chemically. 
  Their 
  relations, 
  moreover, 
  can 
  be 
  

   clearly 
  expressed 
  in 
  terms 
  of 
  a 
  general 
  theory. 
  

  

  In 
  a 
  considerable 
  number 
  of 
  publicationsf 
  issued 
  during 
  the 
  

   past 
  thirty 
  years, 
  I 
  have 
  shown 
  that 
  many 
  of 
  the 
  aluminous 
  

   silicates 
  can 
  be 
  regarded 
  constitutionally 
  as 
  substitution 
  deriva- 
  

   tives 
  of 
  the 
  normal 
  orthosilicate, 
  Al 
  4 
  (Si0 
  4 
  ) 
  3 
  . 
  This 
  conception 
  

   may 
  be 
  modified 
  in 
  form, 
  but 
  not 
  in 
  principle, 
  by 
  treating 
  the 
  

   silicates 
  in 
  question 
  as 
  salts 
  of 
  these 
  alumosilicic 
  acids, 
  

  

  Al 
  3 
  (Si0 
  4 
  ) 
  3 
  H 
  3 
  , 
  trialic 
  acid. 
  

   Al 
  3 
  (Si0 
  4 
  ) 
  3 
  H 
  6 
  , 
  dialic 
  acid. 
  

   Al 
  1 
  (Si0 
  4 
  ) 
  3 
  H 
  9 
  , 
  unalic 
  acid. 
  

  

  the 
  numeral 
  prefixes 
  indicating 
  the 
  number 
  of 
  aluminum 
  atoms 
  

   in 
  each 
  compound. 
  The 
  names 
  are 
  proposed, 
  not 
  as 
  positive 
  

   contributions 
  to 
  nomenclature, 
  but 
  as 
  devices 
  to 
  avoid 
  circum- 
  

  

  *Zeitschr. 
  Kryst., 
  liv, 
  238, 
  1914. 
  

  

  fFor 
  a 
  summary 
  of 
  the 
  whole 
  subject 
  see 
  U. 
  S. 
  Geol. 
  Survey, 
  Bull. 
  588, 
  

   1916. 
  The 
  conclusions 
  there 
  given 
  as 
  to 
  melilite 
  and 
  gehlenite 
  are 
  modified 
  

   in 
  this 
  communication. 
  

  

  