﻿._/". 
  B. 
  U?nj)lehi/, 
  etc. 
  — 
  Custerite 
  : 
  A 
  New 
  Mineral. 
  393 
  

  

  Hillebrandite 
  is 
  genetically 
  similar 
  to 
  custerite, 
  being 
  one 
  of 
  

   the 
  products 
  of 
  contact 
  inetamorphisui 
  of 
  limestone 
  adjacent 
  to 
  

   an 
  igneous 
  mass. 
  A 
  sample 
  of 
  hillebrandite 
  kindly 
  furnished 
  

   for 
  that 
  purpose 
  by 
  Dr. 
  F. 
  E. 
  Wright 
  yielded 
  0*77 
  per 
  cent 
  of 
  

   water 
  at 
  110° 
  and 
  9*64 
  per 
  cent 
  on 
  ignition 
  (calculated 
  945 
  per 
  

   cent). 
  The 
  strong 
  alkaline 
  reaction 
  with 
  phenolphthaline, 
  as 
  

   described 
  by 
  Wright, 
  was 
  confirmed 
  and 
  suggests 
  the 
  presence 
  

   of 
  the 
  CaOH 
  group. 
  On 
  the 
  basis 
  of 
  these 
  results, 
  the 
  com- 
  

   position 
  of 
  hillebrandite 
  can 
  be 
  readily 
  interpreted 
  as 
  a 
  meta- 
  

   silicate 
  with 
  the 
  following 
  structural 
  formula 
  : 
  

  

  bl 
  °3<CaOH 
  

  

  This 
  formula 
  is 
  identical 
  in 
  type 
  with 
  that 
  of 
  custerite, 
  and 
  

   suggests 
  at 
  once 
  that 
  custerite 
  is 
  an 
  isomorphous 
  mixture 
  of 
  

   hillebrandite 
  and 
  a 
  theoretic 
  fl 
  uo-hillebranclite, 
  in 
  which 
  all 
  the 
  

   hydroxyl 
  is 
  replaced 
  by 
  fluorine. 
  This 
  conception 
  is 
  a 
  simple 
  

   and 
  rational 
  one, 
  but 
  is 
  opposed 
  by 
  other 
  considerations: 
  

  

  As 
  cuspidine 
  has 
  been 
  found 
  at 
  two 
  widely 
  separated 
  local- 
  

   ities, 
  the 
  inference 
  may 
  be 
  justified 
  that 
  in 
  the 
  presence 
  of 
  

   much 
  fluorine 
  and 
  little 
  or 
  no 
  water, 
  a 
  mineral 
  of 
  the 
  cuspi- 
  

   dene 
  formula, 
  3Ca0.2Si0 
  Q 
  .CaF 
  o 
  , 
  would 
  always 
  form 
  instead 
  of 
  a 
  

   fluo-hillebrandite 
  with 
  the 
  formula 
  2Ca0.2Si0 
  2 
  .2CaF 
  2 
  . 
  In 
  other 
  

   words, 
  fluo-hillebrandite 
  seems 
  to 
  be 
  unstable 
  under 
  the 
  meta- 
  

   morphic 
  conditions 
  prevailing, 
  judging, 
  however, 
  solely 
  from 
  the 
  

   fact 
  that 
  at 
  the 
  only 
  t^vo 
  localities 
  where 
  a 
  calcium 
  fluo-silicate 
  

   occurs, 
  a 
  different 
  type 
  of 
  compound 
  (cuspidine) 
  was 
  formed. 
  

   It 
  seems 
  likely, 
  therefore, 
  that, 
  considering 
  the 
  isomorphous 
  

   replacement 
  of 
  fluorine 
  and 
  hydroxyl, 
  the 
  isomorphous 
  series 
  

   of 
  which 
  hillebrandite 
  is 
  the 
  hydroxyl 
  end, 
  consists 
  of 
  the 
  two 
  

   end 
  members 
  : 
  (CaOH) 
  s 
  SiO, 
  and 
  (CaF) 
  (CaOH)Si0 
  3 
  and 
  not 
  

   of 
  the 
  theoretic 
  end 
  members 
  : 
  (CaOH) 
  2 
  Si0 
  3 
  and 
  (CaF) 
  2 
  Si0 
  3 
  . 
  

  

  The 
  symmetry 
  of 
  hillebrandite 
  and 
  custerite 
  is 
  apparently 
  

   different, 
  though 
  some 
  of 
  the 
  properties 
  of 
  hillebrandite 
  could 
  

   not 
  be 
  as 
  definitely 
  determined 
  as 
  those 
  of 
  custerite. 
  The 
  

   former 
  mineral 
  is 
  fibrous, 
  orthorhombic, 
  whereas 
  custerite 
  is 
  

   granular, 
  monoclinic, 
  with 
  a 
  close 
  approach 
  to 
  orthorhombic 
  

   symmetry, 
  as 
  evidenced 
  by 
  the 
  nearly 
  rectangular 
  cleavages 
  

   and 
  low 
  extinction 
  angle. 
  The 
  custerite 
  and 
  hillebrandite 
  

   compounds 
  may 
  both 
  be 
  dimorphous, 
  with 
  only 
  the 
  two 
  non- 
  

   isomorphous 
  end 
  members 
  of 
  the 
  four 
  possible 
  compounds 
  

   known. 
  

  

  The 
  status 
  of 
  the 
  relationship 
  of 
  these 
  minerals, 
  as 
  far 
  as 
  can 
  

   be 
  judged 
  by 
  the 
  available 
  evidence, 
  seems 
  to 
  be 
  somewhat 
  as 
  

   is 
  oiven 
  below. 
  

  

  