﻿172 
  

  

  Ford 
  and 
  Bradley 
  — 
  Pyroxmangite. 
  

  

  The 
  analysis 
  yields 
  molecular 
  ratios 
  that 
  agree 
  with 
  the 
  

   accepted 
  type 
  of 
  a 
  pyroxene 
  formula. 
  The 
  small 
  amount 
  of 
  

   water 
  was 
  disregarded. 
  It 
  was 
  probably 
  due 
  to 
  incipient 
  alter- 
  

   ation. 
  The 
  alumina 
  was 
  presumed 
  to 
  be 
  present 
  in 
  the 
  combi- 
  

   nation 
  RO.Al 
  2 
  3 
  .Si0 
  2 
  . 
  After 
  subtracting 
  the 
  proper 
  amounts 
  

   from 
  the 
  silica 
  and 
  protoxide 
  ratios 
  to 
  satisfy 
  this 
  formula, 
  

   the 
  resulting 
  ratios 
  Si0 
  2 
  : 
  RO 
  reduce 
  to 
  1*00 
  : 
  0*917 
  which 
  gives 
  

   the 
  metasilicate 
  formula, 
  RSi0 
  3 
  . 
  The 
  name 
  pyroxmangite 
  

   was 
  given 
  to 
  the 
  mineral 
  in 
  order 
  to 
  indicate 
  that 
  it 
  is 
  a 
  

   manganese 
  pyroxene. 
  

  

  As 
  stated 
  above, 
  there 
  was 
  a 
  black 
  oxide 
  of 
  iron 
  and 
  manga- 
  

   nese 
  intimately 
  associated 
  with 
  the 
  pyroxmangite. 
  The 
  oxide 
  

  

  Fig. 
  2. 
  

  

  is 
  unmistakably 
  an 
  alteration 
  product. 
  It 
  surrounds 
  the 
  

   unaltered 
  silicate, 
  occurring 
  on 
  the 
  outside 
  of 
  the 
  specimens. 
  

   The 
  change 
  from 
  one 
  substance 
  to 
  the 
  other 
  while 
  confined 
  to 
  

   a 
  small 
  space 
  is 
  nevertheless 
  gradual, 
  there 
  being 
  no 
  sharp 
  

   dividing 
  line 
  between 
  the 
  two. 
  In 
  certain 
  instances 
  prominent 
  

   cleavage 
  and 
  parting 
  planes, 
  which 
  were 
  evidently 
  formed 
  

   before 
  the 
  alteration 
  took 
  place, 
  could 
  be 
  traced 
  unbrokenly 
  

   from 
  one 
  mineral 
  into 
  the 
  other. 
  Fig. 
  2 
  represents 
  the 
  change 
  

   as 
  shown 
  in 
  a 
  thin 
  section 
  under 
  the 
  microscope. 
  The 
  alteration 
  

   penetrates 
  the 
  pyroxmangite 
  first 
  along 
  the 
  cleavage 
  cracks. 
  

   The 
  beginning 
  of 
  the 
  alteration 
  is 
  shown 
  in 
  the 
  section 
  by 
  a 
  

   darkening 
  of 
  the 
  color 
  of 
  the 
  silicate 
  to 
  brown, 
  which 
  gradually 
  

   intensifies 
  until 
  the 
  substance 
  becomes 
  black 
  and 
  opaque. 
  

  

  This 
  oxide 
  is 
  metallic 
  in 
  luster, 
  giving 
  a 
  dark 
  chocolate- 
  

   brown 
  streak. 
  It 
  is 
  fusible 
  about 
  4 
  to 
  a 
  black 
  magnetic 
  glob- 
  

   ule. 
  Its 
  hardness 
  is 
  between 
  5 
  # 
  5 
  and 
  trO. 
  When 
  heated 
  in 
  

   the 
  closed 
  tube 
  it 
  yields 
  abundant 
  water 
  and 
  also 
  gives 
  off 
  

  

  