376 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



material along fissures. The so-called talc of the gouges of fissures 

 in the vicinity of the orebodies consists for the most part of sericite. 83 



BIOTITE (462) 



BLACK MICA, IRON MICA 



Variable hydrous silicate of magnesia, Monoclinic. 



iron, alumina, etc., usually approxi- 

 mating the formula (H,K) 2 0. 

 2 (Mg, Fe) O.Al 2 3 .3Si0 2 . 



Biotite is similar in distribution and occurrence to the correspond- 

 ing white mica or muscovite. It occurs similarly in schists and 

 gneisses and is much more frequent as an original constituent of 

 igneous rocks, both acidic and basic. Like muscovite, biotite is 

 frequently found in larger plates and crystals in pegmatites, although 

 no such occurrence in Idaho pegmatites has yet been brought to 

 notice. A number of occurrences are noted below, most of them 

 rather because they have been examined in the course of studying 

 associated minerals than because they possess unusual features. 



CUSTER COUNTY 



Biotite occurs as an original constituent of the granite-porphyry 

 of the Alder Creek district. During endomorphism it has changed to 

 diopside. 84 



LATAH COUNTY 



In the Mizpah mine, Hoodoo district, biotite occurs abundantly 

 as isolated coarse crystals distributed through the solid chalcopyrite 

 and pyrrhotite of the ore. 85 



LEMHI COUNTY 



Biotite is developed in beautiful rosettes in some of the cobalt 

 ores of the Blackbird district, notably those of the Togo claim. 86 

 A specimen from this district collected by Frank L. Hess of the 

 United States Geological Survey, the exact locality for which is not 

 known, consists of large blades of biotite in an indeterminate ser- 

 pentinous matrix. The biotite blades reach a size of 6 by 2 cm. 

 They are dark-greenish brown in color and each blade is divided, 

 centrally, by a vertical line on each side of which are inclined striations 

 giving a feathery appearance. 



A specimen of medium fine granular biotite. from the west end of 

 the Beliel group has been mentioned above as containing tour- 

 maline. Under the microscope this is practically uniaxial and 



83 F. L. Ransome and F. C. Calkins. U. S. Geol. Survey, Prof. Paper 62, 1908. 



8 < J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 50, 1917. 



m D. C. Livingston and F. B. Laney. Idaho Bur. Geol. and Mines, Bull. 1, pp. 95-96, 1920. 



" J. B. Umpleby. U. S. Geol. Survey, Bull. 528, p. 75, 1913. 



