﻿34:4 
  Penfielcl 
  and 
  Warren 
  — 
  Minerals 
  from 
  FranMin^ 
  N. 
  J. 
  

  

  of 
  m/^m^ 
  110/sllO, 
  was 
  found 
  to 
  be 
  47° 
  32', 
  and 
  this 
  angle, 
  

   taken 
  as 
  fundamental, 
  agreed 
  verj 
  closely 
  with 
  the 
  measure- 
  

   ments 
  derived 
  from 
  the 
  best 
  reflections. 
  As 
  terminal 
  planes 
  

   were 
  not 
  observed, 
  the 
  angle 
  between 
  the 
  vertical 
  axes 
  of 
  two 
  

   prisms 
  in 
  twin 
  position 
  was 
  measured 
  under 
  the 
  microscope 
  and 
  

   found 
  to 
  be 
  121°. 
  Assuming 
  the 
  twijining 
  plane 
  to 
  be 
  the 
  

   brachjdome 
  (Oil), 
  the 
  angle 
  of 
  011/^011 
  was 
  thus 
  found 
  to 
  be 
  

   59°, 
  and 
  taken 
  as 
  a 
  fundamental 
  angle. 
  From 
  the 
  foregoing 
  

   fundamental 
  angles 
  the 
  axial 
  ratio 
  has 
  been 
  calculated, 
  and 
  is 
  

   given 
  below, 
  together 
  with 
  the 
  axial 
  ratios 
  of 
  monticellite 
  and 
  

   chrysolite, 
  to 
  which 
  species 
  glaucochroite 
  is 
  closely 
  related, 
  it 
  

   being 
  a 
  manganese 
  monticellite. 
  

  

  Glaucochroite, 
  a 
  : 
  b 
  : 
  c 
  = 
  0*440 
  : 
  1 
  : 
  0-566 
  

   Monticellite, 
  " 
  '' 
  " 
  = 
  0-431 
  : 
  1 
  : 
  0-576 
  

   Chrysolite, 
  " 
  " 
  " 
  = 
  0*466 
  : 
  1 
  : 
  0.586 
  

  

  ISTo 
  reliable 
  reflections 
  could 
  be 
  obtained 
  from 
  the 
  second 
  

   prism 
  s 
  (120). 
  Approximate 
  measurements 
  are 
  120 
  a 
  120 
  = 
  99°, 
  

   calculated 
  97° 
  16' 
  and 
  m^s, 
  110^120, 
  = 
  17° 
  21', 
  calculated 
  

   17° 
  36'. 
  A 
  rather 
  poor 
  basal 
  cleavage 
  was 
  detected, 
  and 
  meas- 
  

   urements 
  from 
  this 
  cleavage 
  onto 
  the 
  prism 
  faces 
  gave 
  angles 
  

   of 
  90°. 
  

  

  The 
  hardness 
  is 
  about 
  6. 
  The 
  specific 
  gravity, 
  taken 
  with 
  

   the 
  pycnometer 
  is 
  3*407. 
  The 
  fracture 
  is 
  conchoidal. 
  The 
  

   luster 
  is 
  vitreous 
  and 
  the 
  color 
  is 
  a 
  delicate 
  bluish 
  green, 
  very 
  

   similar 
  to 
  that 
  of 
  the 
  aquamarine 
  variety 
  of 
  beryl. 
  Minute 
  

   crystals 
  are 
  almost 
  colorless, 
  and 
  on 
  a 
  few 
  of 
  the 
  specimens 
  

   there 
  were 
  small 
  areas 
  where 
  the 
  mineral 
  exhibited 
  a 
  delicate 
  

   pink 
  color. 
  

  

  The 
  optical 
  orientation 
  is 
  a=&, 
  h 
  — 
  c 
  and 
  z 
  — 
  a. 
  The 
  plane 
  

   of 
  the 
  optical 
  axes 
  is 
  the 
  base 
  (001) 
  and 
  the 
  acute 
  bisectrix 
  is 
  

   normal 
  to 
  the 
  brachypinacoid 
  h 
  (010). 
  The 
  double 
  refraction 
  

   is 
  therefore 
  negative. 
  Prismatic 
  crystals 
  served 
  as 
  prisms 
  for 
  

   determining 
  the 
  indices 
  of 
  refraction 
  a= 
  1*686 
  and 
  yS 
  = 
  l*722. 
  

   These 
  values 
  were 
  each 
  derived 
  from 
  the 
  mean 
  of 
  four 
  inde- 
  

   pendent 
  measurements 
  which 
  showed 
  considerable 
  variation, 
  

   owing 
  to 
  the 
  vicinal 
  character 
  of 
  the 
  prismatic 
  faces, 
  but 
  it 
  is 
  

   believed 
  that 
  they 
  represent 
  a 
  close 
  approximation 
  to 
  the 
  true 
  

   values. 
  On 
  a 
  section 
  parallel 
  to 
  the 
  pinacoid 
  (010), 
  which 
  

   measured 
  0*5xl*5"""\ 
  the 
  divergence 
  of 
  the 
  optical 
  axes 
  for 
  

   yellow 
  light, 
  Na 
  flame, 
  was 
  measured 
  on 
  the 
  Fuess 
  axial 
  angle 
  

   apparatus 
  as 
  follows: 
  2 
  E 
  = 
  121° 
  30' 
  and 
  2H 
  in 
  a-mono- 
  

   bromnaphtaIene 
  = 
  63° 
  27'. 
  From 
  these 
  values 
  2yy 
  was 
  found 
  

   to 
  be 
  60° 
  53' 
  and 
  60° 
  49', 
  respectively. 
  The 
  dispersion 
  was 
  

   marked 
  p'^v. 
  From 
  the 
  values 
  a, 
  yS 
  and 
  V, 
  7 
  was 
  calculated 
  

   and 
  found 
  to 
  be 
  1*735. 
  The 
  optical 
  orientation, 
  dispersion 
  and 
  

  

  