﻿386 
  J. 
  B. 
  Umplehy, 
  etc. 
  — 
  Custerite 
  : 
  A 
  New 
  Mineral. 
  

  

  later 
  than 
  the 
  custerite. 
  This 
  view 
  seems 
  to 
  be 
  supported 
  by 
  

   the 
  occurrence 
  of 
  the 
  mineral 
  in 
  the 
  transition 
  zone 
  from 
  

   garnet-diopside 
  rock 
  to 
  marble 
  and 
  its 
  apparent 
  absence 
  about 
  

   the 
  periphery 
  of 
  included 
  limestone 
  blocks 
  where 
  the 
  meta- 
  

   morphic 
  action 
  was 
  most 
  intense. 
  The 
  thin 
  sections 
  and 
  field 
  

   relations 
  therefore 
  suggest 
  that 
  the 
  custerite 
  was 
  formed 
  in 
  

   the 
  outer 
  part 
  of 
  the 
  wave 
  of 
  metamorphism 
  which 
  passed 
  

   from 
  the 
  magma 
  into 
  the 
  limestone. 
  It 
  follows 
  that 
  in 
  any 
  

   given 
  place 
  there 
  appears 
  to 
  have 
  been 
  a 
  rather 
  definite 
  

   sequence 
  of 
  metamorphism 
  from 
  limestone 
  to 
  marble 
  and 
  

   thence 
  through 
  custerite 
  into 
  garnet-diopside-magnetite 
  rock. 
  

   If 
  this 
  sequence 
  held 
  throughout 
  the 
  deposit 
  it 
  is 
  probable 
  

   that 
  the 
  fluorine 
  of 
  the 
  custerite 
  is 
  in 
  part 
  represented 
  by 
  the 
  

   fluorite 
  of 
  the 
  garnet-diopside 
  rock. 
  The 
  field 
  observations 
  on 
  

   the 
  occurrence 
  of 
  custerite 
  in 
  the 
  deposit, 
  however, 
  are 
  so 
  

   incomplete 
  that 
  further 
  speculation 
  as 
  to 
  its 
  genesis 
  might 
  not 
  

   even 
  be 
  suggestive. 
  

  

  Description 
  of 
  Mineral. 
  

  

  Custerite 
  occurs 
  in 
  finely 
  granular 
  masses 
  which 
  may 
  be 
  

   easily 
  mistaken 
  for 
  greenish 
  marble, 
  although 
  the 
  minute 
  

   cleavage 
  faces, 
  which 
  under 
  the 
  hand 
  lens 
  glisten 
  in 
  the 
  sun- 
  

   light, 
  are 
  roughly 
  tabular 
  in 
  shape 
  and 
  chance 
  ones 
  show 
  

   twinning 
  lamellse 
  normal 
  to 
  the 
  elongation. 
  On 
  weathered 
  

   surfaces 
  a 
  chalky 
  crust 
  consisting 
  chiefly 
  of 
  carbonate 
  is 
  not 
  

   uncommon 
  and 
  for 
  a 
  distance 
  of 
  perhaps 
  a 
  millimeter 
  beneath 
  

   it 
  the 
  mineral 
  is 
  white 
  and 
  porcelain-like. 
  

  

  The 
  physical 
  properties 
  of 
  custerite, 
  as 
  determined 
  from 
  the 
  

   hand 
  specimen 
  where 
  individual 
  crystals 
  cannot 
  be 
  isolated, 
  

   are 
  as 
  follows 
  : 
  Hardness, 
  about 
  5 
  ; 
  specific 
  gravity 
  2*91 
  (cor- 
  

   rected* 
  for 
  admixed 
  diopside 
  and 
  magnetite) 
  ; 
  luster, 
  greasy 
  to 
  

   vitreous 
  ; 
  streak, 
  white 
  ; 
  color, 
  pale 
  greenish 
  gray 
  ; 
  tenacity, 
  

   brittle 
  ; 
  translucent. 
  

  

  A 
  microscopic 
  study 
  of 
  thin 
  sections 
  reveals 
  an 
  aggregate 
  of 
  

   irregular, 
  diversely 
  oriented 
  interlocking 
  grains, 
  few 
  of 
  which 
  

   exceed 
  one 
  millimeter 
  in 
  length 
  by 
  a 
  width 
  a 
  little 
  less. 
  Most 
  

   of 
  the 
  grains, 
  however, 
  are 
  about 
  one-half 
  of 
  a 
  millimeter 
  in 
  

   diameter. 
  The 
  mineral 
  has 
  three 
  cleavage 
  directions 
  which 
  

   intersect 
  at 
  angles 
  closely 
  approaching 
  90 
  degrees. 
  The 
  cleav- 
  

   age 
  in 
  each 
  of 
  the 
  three 
  directions 
  is 
  so 
  interrupted 
  that 
  it 
  is 
  

   impossible 
  to 
  measure 
  accurately 
  their 
  angles 
  of 
  intersection. 
  

   In 
  two 
  of 
  the 
  directions 
  the 
  cleavage 
  is 
  about 
  equally 
  promi- 
  

   nent 
  but 
  in 
  the 
  other 
  it 
  is 
  more 
  nearly 
  perfect. 
  Polysynthetic 
  

   twinning 
  is 
  beautifully 
  developed 
  parallel 
  to 
  this 
  principal 
  

   cleavage. 
  The 
  lamellae 
  are 
  seldom 
  wider 
  than 
  001 
  millimeter 
  

   and 
  are 
  commonly 
  much 
  narrower 
  but 
  are 
  not 
  uniformly 
  dis- 
  

  

  * 
  Value 
  obtained 
  =2 
  -96. 
  

  

  