﻿& 
  L. 
  Penfield 
  — 
  Graftonite, 
  a 
  new 
  Mineral. 
  27 
  

  

  problem 
  presented 
  by 
  this 
  particular 
  intergrowth, 
  however, 
  it 
  

   may 
  be 
  well 
  to 
  cite 
  a 
  few 
  illustrations 
  of 
  a 
  somewhat 
  similar 
  

   nature. 
  The 
  best 
  and 
  undoubtedly 
  the 
  most 
  familiar 
  illustra- 
  

   tion 
  of 
  the 
  lamellar 
  intergrowth 
  of 
  two 
  minerals 
  is 
  that 
  of 
  

   albite 
  (triclinic) 
  and 
  orthoclase 
  (monoclinic), 
  a 
  phenomenon 
  

   commonly 
  observed 
  in 
  studying 
  thin 
  sections 
  of 
  rocks, 
  and 
  

   familiarly 
  known 
  to 
  petrographers 
  as 
  the 
  perthitic 
  or 
  micro- 
  

   perthitic 
  structure. 
  In 
  the 
  case 
  of 
  the 
  feldspars 
  the 
  two 
  

   minerals 
  are 
  closely 
  related 
  both 
  in 
  chemical 
  composition 
  and 
  

   crystalline 
  form, 
  they 
  are 
  united 
  by 
  their 
  pinacoid 
  faces 
  b, 
  010, 
  

   having 
  their 
  vertical 
  axes 
  parallel, 
  and 
  thus 
  the 
  cleavage 
  

   directions 
  and 
  the 
  interfacial 
  angles 
  of 
  the 
  two 
  minerals 
  con- 
  

   form 
  very 
  closely 
  to 
  one 
  another. 
  Other 
  familiar 
  instances 
  of 
  

   parallel, 
  though 
  not 
  of 
  lamellar 
  intergrowth 
  are 
  of 
  staurolite 
  

   (orthorhombic) 
  on 
  cyanite 
  (triclinic), 
  rutile 
  (tetragonal) 
  on 
  

   hematite 
  (hexagonal-rhombohedral), 
  and 
  chalcopyrite 
  (tetra- 
  

   gonal) 
  on 
  both 
  tetrahedrite 
  and 
  sphalerite 
  (isometric). 
  In 
  each 
  

   of 
  these 
  cases 
  it 
  is 
  probable 
  that 
  the 
  two 
  minerals 
  have 
  certain 
  

   molecular 
  or 
  crystallographic 
  features 
  so 
  nearly 
  alike 
  that 
  the 
  

   two 
  substances 
  can 
  grow 
  together 
  in 
  certain 
  definite 
  positions, 
  

   somewhat 
  analogous 
  to 
  the 
  growth 
  of 
  two 
  crystals 
  of 
  the 
  same 
  

   material 
  in 
  parallel 
  or 
  in 
  twin 
  position. 
  

  

  Evidently 
  the 
  most 
  important 
  thing 
  to 
  be 
  determined 
  in 
  

   connection 
  with 
  the 
  study 
  of 
  the 
  interlamination 
  of 
  the 
  mono- 
  

   clinic 
  graftonite 
  and 
  the 
  orthorhombic 
  triphylite 
  is 
  the 
  manner 
  

   in 
  which 
  the 
  two 
  minerals 
  are 
  intergrown. 
  Undoubtedly 
  the 
  

   graftonite, 
  which 
  makes 
  up 
  from 
  two-thirds 
  to 
  three-quarters 
  

   of 
  the 
  total 
  mass 
  of 
  the 
  crystals, 
  has 
  exerted 
  the 
  controlling 
  

   influence 
  upon 
  the 
  crystalline 
  form, 
  for 
  the 
  external 
  shape 
  of 
  

   the 
  crystals 
  is 
  monoclinic 
  and 
  not 
  orthorhombic. 
  In 
  all 
  of 
  the 
  

   specimens 
  the 
  lamination 
  is 
  very 
  pronounced 
  and 
  parallel 
  to 
  

   the 
  clinopinacoid 
  b, 
  010, 
  of 
  the 
  graftonite 
  ; 
  however, 
  the 
  sur- 
  

   faces 
  by 
  which 
  the 
  two 
  minerals 
  are 
  united 
  are 
  not 
  plane 
  and 
  

   even, 
  like 
  cleavage 
  or 
  crystal 
  faces, 
  but 
  undulatory, 
  somewhat 
  

   resembling 
  in 
  miniature 
  a 
  surface 
  presented 
  by 
  a 
  slab 
  of 
  sedi- 
  

   mentary 
  rock 
  exhibiting 
  ripple 
  marks. 
  An 
  idea 
  of 
  the 
  uneven 
  

   character 
  of 
  the 
  surfaces 
  may 
  be 
  gained 
  by 
  noting 
  the 
  irregular, 
  

   wavy 
  lines 
  of 
  juncture 
  between 
  the 
  two 
  minerals 
  as 
  seen 
  in 
  cross 
  

   section, 
  fig. 
  1. 
  In 
  the 
  specimen 
  represented 
  by 
  fig. 
  2 
  there 
  

   was 
  a 
  limited 
  area 
  where 
  the 
  lamination 
  was 
  apparently 
  parallel 
  

   to 
  the 
  prism 
  m 
  of 
  the 
  graftonite, 
  but 
  in 
  other 
  parts 
  of 
  the 
  

   specimen 
  and 
  in 
  all 
  other 
  crystals 
  the 
  lamination 
  was 
  through- 
  

   out 
  parallel 
  to 
  the 
  clinopinacoid 
  b. 
  

  

  When 
  a 
  section 
  like 
  fig. 
  1 
  is 
  examined 
  in 
  polarized 
  light, 
  

   the 
  graftonite, 
  as 
  already 
  stated, 
  shows 
  parallel 
  extinction 
  ; 
  the 
  

   extinction 
  of 
  the 
  triphylite, 
  however, 
  in 
  the 
  particular 
  section 
  

   under 
  consideration, 
  is 
  inclined 
  about 
  27° 
  to 
  the 
  direction 
  of 
  

  

  