﻿26 
  

  

  & 
  L. 
  Penfield 
  — 
  Graftonite, 
  a 
  new 
  Mineral. 
  

  

  ments 
  indicated 
  the 
  simple 
  forms 
  which 
  have 
  been 
  recorded 
  in 
  

  

  the 
  foregoing" 
  list. 
  

  

  The 
  optical 
  properties 
  of 
  the 
  graftonite, 
  as 
  far 
  as 
  they 
  were 
  

   studied, 
  indicate 
  monoclinic 
  symmetry. 
  The 
  lamination, 
  as 
  

   will 
  be 
  explained 
  more 
  in 
  detail 
  later, 
  is 
  parallel 
  to 
  the 
  pinacoid 
  

   b, 
  010, 
  of 
  the 
  graftonite, 
  and 
  hence 
  a 
  section 
  like 
  that 
  repre- 
  

   sented 
  by 
  fig. 
  1, 
  cut 
  at 
  right 
  angles 
  to 
  the 
  lamination, 
  is 
  at 
  

  

  right 
  angles 
  to 
  the 
  symmetry 
  plane. 
  Two 
  sections 
  were 
  thus 
  

   prepared 
  from 
  the 
  small 
  fragment 
  of 
  fresh 
  material, 
  and, 
  

   except 
  that 
  they 
  were 
  at 
  right 
  angles 
  to 
  the 
  symmetry 
  plane, 
  

   nothing 
  is 
  known 
  concerning 
  their 
  orientation. 
  In 
  these 
  

   sections, 
  when 
  examined 
  in 
  polarized 
  light, 
  the 
  graftonite 
  was 
  

   found 
  to 
  have 
  an 
  extinction 
  parallel 
  to 
  the 
  lamination, 
  as 
  indi- 
  

   cated 
  by 
  the 
  arrows, 
  fig. 
  1. 
  In 
  one 
  of 
  the 
  sections, 
  when 
  

   examined 
  in 
  convergent 
  polarized 
  light, 
  a 
  portion 
  of 
  a 
  biaxial 
  

   interference 
  figure 
  was 
  observed 
  toward 
  the 
  limit 
  of 
  the 
  field, 
  

   and 
  the 
  position 
  of 
  the 
  figure 
  indicated 
  distinctly 
  that 
  the 
  plane 
  

   of 
  the 
  optical 
  axes 
  is 
  at 
  right 
  angles 
  to 
  the 
  symmetry 
  plane, 
  

   the 
  acute 
  bisectrix 
  being 
  in 
  the 
  symmetry 
  plane. 
  By 
  mount- 
  

   ing 
  a 
  small 
  fragment 
  of 
  the 
  thin 
  section 
  on 
  the 
  little 
  axial 
  

   angle 
  apparatus 
  devised 
  by 
  .Professor 
  Klein,* 
  filling 
  the 
  vessel 
  

   with 
  a 
  liquid 
  of 
  high 
  index 
  of 
  refraction, 
  and 
  turning 
  until 
  the 
  

   bisectrix 
  came 
  to 
  the 
  center 
  of 
  the 
  field 
  of 
  the 
  polariscope, 
  the 
  

   horizontal 
  character 
  of 
  the 
  dispersion 
  could 
  be 
  distinctly 
  seen, 
  

   and 
  it 
  was 
  estimated 
  that 
  the 
  divergence 
  of 
  the 
  optical 
  axes, 
  

   2Y, 
  was 
  about 
  50° 
  to 
  60°. 
  The 
  birefringence 
  is 
  not 
  very 
  

   strong 
  and 
  its 
  character 
  is 
  positive. 
  

  

  The 
  interlamination 
  of 
  two 
  minerals 
  such 
  as 
  graftonite 
  and 
  

   triphylite, 
  differing 
  in 
  chemical 
  composition 
  and 
  crystallizing 
  

   in 
  different 
  systems, 
  is 
  an 
  interesting 
  and 
  unusual 
  phenomenon 
  

   which 
  seemed 
  worthy 
  of 
  some 
  study. 
  Before 
  discussing 
  the 
  

  

  ♦Groth's 
  Physikalische 
  Krystallographie, 
  1895, 
  fig. 
  690, 
  p. 
  750. 
  

  

  