136 F. W. Clarke — /Studies in the Mica Group. 



Upon microscopic examination by Mr. Diller, the mica was 

 found to be " almost completely uniaxial, for when it was 

 rotated between crossed nicols the displacement of the black 

 cross was scarcely perceptible." He also found that the same 

 mica, with a slight change of color from brown to greenish 

 brown, continued into the border; where it was intimately 

 mingled with grains and fibers of quartz. The latter admixture 

 accounts in part for the high percentage of silver in the margin ; 

 although it seems probable that there may be also some 

 degradation from the original orthosilicate toward a metasili- 

 cate type. At all events the specimens are in nowise analogous 

 to those which show lepidolite borders upon muscovite. 



In its ratios this Auburn mica is not simple. Approximately, 

 reducing all monoxides to the univalent type, its formula may 

 be written R / 11 (A10) 3 Al 2 (Si0 4 ) 6 , ; with R'^HJK^Fe^. It seems 

 to be a compound intermedial e between the Litchfield and 

 Baltimore micas, with R" 7 nearly all aluminum. 



4. Iron- Mica from near Pikers Peak. 



This specimen, which was kindly given me by Mr. C. S. 

 Bement, was a section from a large prismatic crystal in his col- 

 lection. It was a bronzy black mica, resembling phlogopite 

 externally, but remarkably altered at the center. The entire 

 core of the crystal was made up of a soft, rotten material, evi- 

 dently derived from the original mica, and surrounded by a 

 broad, black margin of the latter. Streaks of rusty alteration 

 products reached into the margin in every direction, so that no 

 absolutely unchanged material could be obtained for analysis. 

 According to Mr. Diller, the angle of the optic axes is too 

 small for measurement. 



Analyses were made of both margin and center, the former 

 being as little altered, the latter as much altered as could be 

 selected. 



Again I am indebted to Mr. Riggs for determinations of water 

 and ferrous iron : 



Margin. Center. 



H 2 4-54 Ignition 



Si0 2 34-21 



A1 2 3 16-53 



Fe 2 3 20-15 



FeO 14-17 



MnO -91 



CaO..." -48 



MgO 1-34 



Na 2 1-43 



K„0._ 6-50 



7-82 



34-63 



17-95 



31-25 



3-01 



•34 



•81 



1-08 



•89 



1-96 



