﻿OF 
  THE 
  DIAMANTIFEROTJS 
  ROCK 
  OF 
  SOUTH 
  AFRICA. 
  411 
  

  

  it 
  will 
  be 
  seen 
  that 
  they 
  do 
  not 
  coincide 
  with 
  those 
  of 
  either 
  of 
  these 
  

  

  minerals. 
  

  

  RO. 
  R 
  2 
  3 
  . 
  Si0 
  2 
  . 
  H„0. 
  

  

  Jefterisite 
  12 
  6 
  15 
  18 
  

  

  Hallite 
  12 
  2 
  9 
  12 
  

  

  Vaalite 
  12 
  2 
  10 
  8 
  

  

  As 
  we 
  can 
  see 
  no 
  grounds 
  for 
  identifying 
  the 
  mineral 
  under 
  

   description 
  with 
  either 
  of 
  the 
  above, 
  we 
  propose 
  to 
  call 
  it 
  Vaalite, 
  

   from 
  the 
  Vaal 
  river, 
  and 
  to 
  consider 
  it 
  to 
  be 
  an 
  outlying 
  member 
  of 
  

   the 
  vermiculite 
  family. 
  

  

  Analyses 
  are 
  given 
  in 
  the 
  paper 
  of 
  Prof. 
  Cooke 
  of 
  two 
  varieties 
  

   of 
  Hallite, 
  a 
  green 
  and 
  a 
  yellow, 
  which 
  vary 
  very 
  slightly 
  in 
  compo- 
  

   sition, 
  and 
  are 
  only 
  distinguished 
  by 
  the 
  fact 
  of 
  the 
  green 
  variety 
  

   containing 
  somewhat 
  more 
  than 
  1 
  per 
  cent. 
  (1*13 
  per 
  cent.) 
  of 
  iron 
  

   protoxide, 
  and 
  the 
  yellow 
  variety 
  0-32 
  per 
  cent. 
  This 
  would 
  lead 
  

   to 
  the 
  assumption 
  that 
  the 
  development 
  of 
  colour 
  is 
  due 
  to 
  the 
  

   presence 
  of 
  a 
  small 
  amount 
  of 
  iron 
  in 
  the 
  lower 
  state 
  of 
  oxidation. 
  

   The 
  drab 
  vaalite 
  is 
  accompanied, 
  as 
  has 
  already 
  been 
  stated, 
  by 
  a 
  

   few 
  flakes 
  of 
  a 
  fine 
  blue 
  tint 
  ; 
  and 
  it 
  was 
  considered 
  important 
  to 
  

   determine 
  whether 
  these 
  contained 
  iron 
  protoxide. 
  They 
  were 
  

   boiled 
  with 
  strong 
  acid 
  in 
  an 
  atmosphere 
  of 
  carbonic 
  acid, 
  and 
  tested 
  

   with 
  gold 
  solution, 
  when 
  a 
  precipitation 
  of 
  metal 
  equivalent 
  to 
  

   4*282 
  per 
  cent, 
  of 
  iron 
  protoxide 
  took 
  place. 
  

  

  2. 
  Bronzite. 
  

  

  Small 
  bright 
  green 
  crystals, 
  with 
  something 
  of 
  an 
  emerald 
  tint, 
  

   are 
  among 
  the 
  minerals 
  that 
  occur 
  in 
  the 
  diamantiferous 
  rocks 
  of 
  

   Du 
  Toit's 
  Pan; 
  and 
  they 
  are 
  at 
  the 
  same 
  time 
  a 
  permanent 
  in- 
  

   gredient, 
  more 
  particularly 
  in 
  the 
  neighbourhood 
  of 
  the 
  garnet, 
  at 
  

   Bultfontein, 
  and 
  occur 
  more 
  or 
  less 
  sparsely 
  in 
  the 
  rock 
  of 
  Coles 
  - 
  

   berg 
  Kopje. 
  

  

  These 
  crystals 
  resemble 
  very 
  closely 
  those 
  of 
  the 
  bronzite 
  of 
  the 
  

   meteorite 
  of 
  Breitenbach, 
  but 
  are 
  less 
  rich 
  in 
  crystallographic 
  planes 
  ; 
  

   in 
  fact 
  they 
  are 
  nearly 
  confined 
  to 
  those 
  of 
  the 
  prism 
  and 
  of 
  the 
  faces 
  

   of 
  the 
  form 
  100. 
  The 
  prism-angle 
  is 
  about 
  87° 
  20', 
  and 
  the 
  prin- 
  

   cipal 
  sections 
  are 
  parallel 
  and 
  pei*pendicular 
  to 
  the 
  zone 
  001. 
  The 
  

   crystal 
  is 
  undoubtedly 
  prismatic. 
  

  

  It 
  was 
  dried 
  at 
  120° 
  C., 
  and 
  analyzed 
  with 
  the 
  following 
  results 
  : 
  — 
  

  

  Oxygen 
  Ratios. 
  

  

  Silicic 
  acid 
  55-908 
  29-817 
  

  

  Alumina 
  2*638 
  

  

  Chromium 
  oxide 
  0*539 
  

  

  Iron 
  protoxide 
  4*991 
  1*109 
  1 
  

  

  Nickel 
  oxide 
  trace 
  I 
  15*073 
  

  

  Magnesia 
  34*912 
  13*964 
  J 
  

  

  Lime 
  0*457 
  

  

  Water 
  not 
  determined 
  

  

  99*445 
  

  

  2e2 
  

  

  