THE MINERALS OF IDAHO 



343 



and structure and greenish-black in color. Under the microscope 

 the most abundant mineral is mica, apparently high-iron biotite 

 which is practically uniaxial and negative with e= 1.603, co = 1.655, 

 both ±.003. It is pleochroic in plates on edge with « = deep brown- 

 ish olive-green, e = pale brown, Absorption a>> >e. Next in abun- 

 dance is tourmaline, which is unevenly distributed in the mass of 

 the specimen. The tourmaline is uniaxial and negative with the 

 indices e= 1.627, w = 1.650. It is very pleochroic with e = very pale 

 lilac brown, co = deep indigo to blackish-blue. Absorption co>>€. 

 The third mineral of the aggregate is colorless and breaks into flat 

 plates like a mica and these, when on edge, show a high birefringence. 

 This mineral is biaxial with 2V small to 

 medium, and a mean index of refraction, 

 /3 = 1.598. It may be muscovite. 



SHOSHONE COUNTY 



In Shoshone County in the Coeur d'Alene 

 district tourmaline is a microscopic constituent 

 of the sedimentary rocks where it occurs in 

 grains and imperfectly terminated prisms. 

 Mr. Calkins, who has studied the occurrence of 

 tourmaline in the area at large, regards it as a 

 secondary constituent and not, like zircon, a 

 mechanical inclusion in the sediments. It is 

 never found in waterworn grains and, like 

 siderite, it is most abundant where the rocks 

 have been most disturbed. It does not, how- 

 ever, exhibit any such close relation to the 

 monzonite intrusion as to lead to its classifi- 

 cation as an ordinary contact-metamorphic 

 mineral. As a rule tourmaline crystals are 

 most numerous and best developed in the 

 finer-grained sediments, particularly those hav- 

 ing an abundant sericitic matrix. The coarse 



highly siliceous quartzites and the calcareous rocks of the Wallace 

 formation are apparently the least favorable to the development of 

 the mineral. 66 



Ordinary black tourmaline occurs in a number of places in the 

 Avery quadrangle in southern Shoshone County. Specimens of 

 white anorthosite from the southeastern part of the quadrangle have 

 crystals and rosettes of brownish black tourmaline up to 2 cm. in 

 diameter on joints. A specimen of fine-grained quartz from a pros- 

 pect at the mouth of the creek east of Bluff Creek contains scattered 

 grains and masses of black tourmaline. A specimen labeled pegmatite 

 from a locality 2]^> miles south of Trimmed Tree consists mainly of 



b 



h 



Fig. 108.— Tourmaline . 

 Avery Quadrangle, 

 Shoshone County 



F. L. Ransome and F. C. Calkins. U. S. Geol. Survey., Prof. Paper 02, p. 101, 1908. 



