254 



PROCEEDINGS OF THE ACADEMY OF 



[1880. 



ON SIDEEOPHYLLITE— A NEW MINERAL. 

 BY HENRY CARVILL LEWIS. 



A.mong other interesting minerals which are found in the neigh- 

 borhood of Pike's Peak, Colorado, is a hard black mica, occurring 

 sometimes in large and fine crj'Stals, which the writer has been 

 unable to identify with any known species. 



It is monoclinic, and has an eminent micaceous basal cleavage. 

 It has the following characters : 



Hardness, 3.2. Specific gravit}^, 3.1. Lustre, bright micaceous. 

 Color, black b}- reflected light, and fine chrome-green by trans- 

 mitted light. Opaque except in very thin pieces. Streak, pale 

 green. Laminae very brittle. Biaxial ; optic-axial divergence 10° ±. 



In its composition it appears to be an iron-alumina mica. The 

 analysis here given is a mean of two made b}^ the writer. In one 

 the mineral was fused with sodic carbonate before solution, and in 

 the other it was dissolved in hydrochloric acid. The anal3'ses 

 were performed in the usual way. Iron was estimated by solution 

 in sulphuric acid in a closed flask, and subsequent titration. The 

 percentage of alkalies was kindly determined by Mr. F. A. Genth, 

 Jr. The percentage of water is that given off on moderate igni- 

 tion. On strong ignition the mineral loses over 3 per centum of 

 its weight, some of the alkalies being driven off. 



ratio. 



SiO, 



A 1,0s 



Fe,0:, 



PeO 



MnO 



MgO 



CaO 



Na.,0 



Li,0 



K.,0 



H,0 



36.68 



20.41 



1.55 



25.50 



2.10 



1.14 



.81 



1.09 



.37 



9.20 



1.01 



2.44 

 1.19 I 



.06 j 



.n 



.06 

 .06 

 .03 

 .03 

 .02 

 .20 

 .11 



2.44 

 1.25 



1.22 



2.00 

 1.02 



1.00 



This gives R 



99.86 

 R : Si = 1 



Silica 1 

 basic. 



1:2, and for the ratio of bases to 

 1. It is therefore a Unisilicate in which the water is 



