THE MINEKALS OF IDAHO 377 



negative with <^=1.603, and w = 1.655, both ±0.003. Plates on edge 

 show pleochroism with co = deep brownish olive-green, e = pale 

 brown. Absorption co>>e. Associated with the biotite of this 

 specimen is some muscovite as well as a considerable amount of 

 tourmaline in microscopic crystals. 



SHOSHONE COUNTY 



A peculiar green biotite occurs in microscopic grains in the ores of 

 the Success and other mines surroundino- the monzonite area of 

 Ninemile Creek. 87 Biotite occurs in gol v i brown to black scales 

 up to 1 cm. in diameter and often with sharp hexagonal outlines, 

 in lamprophyric rocks at several places in the district. 88 In the 

 metamorphic rocks in the southern part of the county, particularly 

 in the area included in the southern part of the Avery quadrangle, 

 biotite is a common product of the metamorphism of the Belt sedi- 

 mentary rocks, both in the form of schists containing coarse flakes 

 of biotite and as fine purplish hornfels which consist largely of micro- 

 scopic biotite. Such rocks, derived by the metamorphism of the 

 Wallace formation, have been described as containing hornblende 

 and scapolite. Bordering many small pegmatitic intrusions and 

 larger granitic dikes in the biotitic metamorphic rocks there appear 

 considerable layers of biotite in schistose aggregates of coarse brown 



to black plates. 



CHLORITOID (466) 



BEITTLE MICA, MASONITE 



Hydrous silicate of magnesia, lime, Monoclinic. 



iron, alumina, etc., approximating 

 the formula: (Mg,Ca)O.Al 2 3 . 

 Si0 2 .H 2 0. 



Chloritoid is a relatively uncommon mineral, like the other mem- 

 bers of its group. It is the only one of the so-called brittle-mica 

 minerals thus far found in Idaho. 



LEMHI COUNTY 



Chloritoid occurs as the principal constituent of a lot of material 

 which, according to the label, is typical of a contact-metamorphic 

 mass occurring on the hanging wall side of a dike about 10 fort wide 

 on the Nickel Plate and the adjoining property in the Blackbird 

 district. 



In the hand specimen this material is coarse granular and dark 

 grayish-green in color and is made up of interlocking crystals of the 

 chloritoid averaging a little less than 1 cm. in diamteer. Associated 



" F. L. Ransome. U. S. Geol. Survey, Prof. Paper G2, p. 101, 1908. 



8» Earl V. Shannon. Petrography of some lamprophyric dike rocks of the Coeur d'Alene mining district, 

 Idaho. Proc. U. S. National Museum, vol. 57, pp.475-495, 1920. 



54347— 26t 25 



