122 RE-EXAMINATION OF AMERICAN MINERALS. 



15. Greenwood Mica — Biotite. 



The chemical constitution of only a very few American 

 biotites has been examined. In fact, von Kobell's* analysis 

 of a mica from Monroe, N. Y., is the only one that has been 

 published; unfortunately, even in regard to this there is some 

 doubt as to its locality, for biotite is found in more than one 

 place near Monroe. 



The specimens we have examined are from Greenwood Fur- 

 nace, Monroe, N. Y. ; the mineral occurs in large crystals of a 

 dark olive-green color, and the results of the analyses are such 

 as to lead to the supposition that the specimens examined by 

 von K obeli were from this locality. He obtained 



Si £1 ffe Mg K HF Ti ft 



40.00 16.16 7.50 21.54 10.83 0.53 0.20 3.00=99.76 



The results of our analyses are 



1 2 



Silica 39.88 39.51 



Alumina 14.99 15.11 



Peroxide of iron 7.68 7.99 



Magnesia 23.69 23.40 



Potash 9.111 1090 



Soda 1.12 f WZU 



"Water 1.30 1.35 



Fluorine 95 .95 



Chlorine 44 .44 



99.16 98.95 



These give the oxygen ratio 



R & Si 



1 11.31 : 9.31 : 20.72 



2. 11.20: 9.45:20.53 



or very nearly 1:1:2, which corresponds to the formula 

 S 8 {§i4-Bj Si. A small portion of the oxygen in the mineral is 

 replaced by fluorine and chlorine. 



The specimens examined were received from Messrs. Jenkins 

 & Horton, of Monroe, N. Y. 



16. Biotite of Putnam County, ~N. Y. 



In appearance this mineral resembles talc, having a wavy, 

 lamellar structure and a soapy feel. Its color is brownish- 



* J. f. pr. Chem., xxxvi, 309, and Dana's Mineralogy, third edition, p. 360. 



