0. H. Warren — Alteration of Augite- Ilmenite Groups. 473 



large size and possessed of a strong, bright blue pleochroism, 

 parallel to C. Near the borders of the aggregates they are 

 more abundantly developed, in fact they not infrequently 

 form a distinct border of blue prisms jutting out into the 

 plagioclase. Crystals of hornblende having a yellowish to 

 reddish brown pleochroism parallel to C occur with the blue 

 and appear to have been formed in the same way, indeed the 

 same crystal may show a blue pleochroism in one portion and a 

 brown in another. Both the blue and the brown variety may 

 also be seen, changed, probably by a bleaching process, to a 

 nearly colorless variety. Within the aggregates there appears 

 to be some actinolite. 



The biotite is of a light brown color and its formation is 

 clearly connected with the presence of ilmenite. It occurs in 

 part in the form of flakes or shreds mingled with the horn- 

 blende. Its amount varies, being most abundant in the 

 neighborhood of the ilmenite, where it not infrequently prac- 

 tically displaces the hornblende. Its most striking mode 

 of occurrence is, however, in the form of a clearly marked 

 border lying between the hornblende and the plagioclase. 

 These borders as a rule practically surround the entire altered 

 area. Their width, relative to that of the area enclosed, varies 

 considerably in different cases but is always large (perhaps 

 from 1/5 to 1/12). The border consists essentially of narrow 

 shreds or flakes, orientated perpendicularly to the contact with 

 the feldspar, into which they penetrate quite irregularly. Blue 

 hornblende (rarely brown) prisms frequently make their appear- 

 ance in these biotite rims, and, in places, as noted above, may 

 actually constitute a rim themselves. 



Needles and stouter prisms of hornblende often accom- 

 panied by biotite are abundantly developed along fractures 

 and cleavages in the feldspars, while minute isolated horn- 

 blende crystals are to some extent disseminated in many of the 

 plagioclase crystals (gewanderte hornblende). A small amount 

 of calcite and limonite is also present. 



Both biotite and hornblende have been noted lying directly 

 in contact with the ilmenite ; of the two, biotite seems, how- 

 ever, to be much the most intimately connected as regards 

 formation with the ilmenite. It sometimes forms a continuous 

 crystal about a portion of or even an entire ore grain. In 

 such cases the biotite often contains a considerable amount of 

 included material in the form of minute grains, for the most 

 part opaque but sometimes feebly translucent. Some which 

 show a reddish color are probably rutile. These particles are 

 undoubtedly residual since every gradation between biotite 

 with only a small amount of included matter and that contain- 

 ing a core of massive, unchanged ilmenite, may be seen. It 



