﻿452 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  20 
  

  

  shows 
  the 
  rutile-ilmenite 
  relations 
  referred 
  to 
  above 
  to 
  be 
  (1) 
  definite 
  

   intergrowths 
  of 
  the 
  two 
  minerals, 
  and 
  (2) 
  alteration 
  of 
  rutile 
  to 
  ilmen- 
  

   ite, 
  all 
  the 
  important 
  stages 
  in 
  the 
  process 
  being 
  traced 
  under 
  the 
  mi- 
  

   croscope. 
  That 
  the 
  individual 
  crystals 
  in 
  most 
  instances 
  are 
  composed 
  

   of 
  a 
  mixture 
  of 
  the 
  two 
  minerals 
  is 
  also 
  evident 
  megascopically, 
  but 
  

   without 
  the 
  microscope 
  it 
  is 
  not 
  possible 
  to 
  distinguish 
  between 
  ilmen- 
  

   ite 
  that 
  is 
  primary 
  and 
  that 
  which 
  is 
  secondary. 
  

  

  Much 
  of 
  the 
  ilmenite 
  is 
  undoubtedly 
  secondary, 
  the 
  early 
  stages 
  in 
  

   the 
  alteration 
  of 
  the 
  rutile 
  being 
  particularly 
  well-marked 
  under 
  the 
  

   microscope. 
  These 
  are 
  shown 
  principally 
  in 
  irregular 
  dark 
  gray 
  

   nearly 
  black 
  patches 
  and 
  areas 
  of 
  variable 
  size, 
  composed 
  of 
  exceed- 
  

   ingly 
  fine 
  granules 
  of 
  ilmenite, 
  not 
  connected 
  apparently 
  with 
  lines 
  of 
  

   fracture 
  or 
  parting; 
  and 
  in 
  similar 
  irregular 
  areas, 
  directly 
  connected 
  

   with 
  both 
  fracture 
  and 
  parting 
  (cleavage) 
  planes, 
  which 
  gradually 
  

   fade 
  into 
  the 
  unaltered 
  rutile. 
  In 
  each 
  case 
  the 
  boundaries 
  are 
  grada- 
  

   tional 
  and 
  lack 
  sharpness 
  of 
  definition. 
  Some 
  ilmenite 
  is 
  developed 
  

   in 
  the 
  rutile 
  in 
  reasonably 
  closely 
  spaced 
  parallel 
  line-like 
  bodies 
  

   having 
  sharp 
  definition 
  and 
  strongly 
  suggestive 
  of 
  primary 
  origin, 
  but 
  

   they 
  may 
  possibly 
  be 
  secondary. 
  

  

  A 
  part 
  of 
  the 
  ilmenite 
  on 
  the 
  other 
  hand 
  is 
  definitely 
  primary, 
  the 
  

   two 
  minerals, 
  rutile 
  and 
  ilmenite, 
  being 
  molded 
  against 
  each 
  other 
  in 
  

   sharp 
  and 
  distinct 
  contacts, 
  and 
  each 
  is 
  inclosed 
  in 
  the 
  other 
  as 
  in 
  si- 
  

   multaneous 
  crystallization. 
  In 
  some 
  thin 
  sections, 
  the 
  boundaries 
  are 
  

   more 
  angular 
  and 
  irregular 
  and 
  are 
  marked 
  by 
  reentrances 
  and 
  tiny 
  

   stringers 
  of 
  ilmenite 
  penetrating 
  the 
  rutile. 
  In 
  such 
  cases 
  these 
  rela- 
  

   tions 
  may 
  cast 
  doubt 
  on 
  the 
  primary 
  character 
  of 
  the 
  ilmenite, 
  but 
  

   the 
  irregular 
  angular 
  boundaries 
  of 
  the 
  rutile-ilmenite 
  are 
  no 
  more 
  

   emphasized 
  than 
  are 
  the 
  boundaries 
  between 
  rutile 
  and 
  quartz 
  in 
  the 
  

   same 
  thin 
  sections. 
  

  

  Some 
  of 
  the 
  ilmenite 
  has 
  undergone 
  partial 
  alteration 
  about 
  the 
  

   borders 
  and 
  along 
  fractures 
  to 
  a 
  fine-granular 
  grayish 
  substance 
  

   probably 
  leucoxene. 
  Such 
  peripheral 
  alteration 
  of 
  the 
  ilmenite 
  is 
  

   shown 
  whether 
  in 
  contact 
  with 
  rutile 
  or 
  not. 
  Distributed 
  through 
  

   this 
  alteration 
  product 
  at 
  times 
  are 
  very 
  minute 
  black 
  granules 
  which 
  

   may 
  represent 
  residual 
  ilmenite, 
  although 
  their 
  identity 
  is 
  uncertain. 
  

   The 
  alteration 
  in 
  such 
  cases 
  makes 
  it 
  uncertain 
  as 
  to 
  whether 
  the 
  il- 
  

   menite 
  was 
  originally 
  primary 
  or 
  secondary, 
  but 
  the 
  writer 
  is 
  rather 
  

   inclined 
  to 
  regard 
  much 
  of 
  it 
  as 
  having 
  been 
  originally 
  primary 
  and 
  

   subsequently 
  altered. 
  

  

  