THE MINERALS OF IDAHO 379 



of ferrous and ferric iron, alumina, and magnesia. In composition 

 and structure the} 7 " are related to the micas and they almost invariably 

 possess a coarse or fine micaceous structure. A majority of the 

 members of the group are characterized by a green color. 



The following chlorites from Idaho localities have been classified 

 as accurately as possible from the data available, but many of the 

 identifications are but tentative. Few of the examinations could be 

 completed on account of scarcity of material. 



ADAMS COUNTY 



Chlorite is listed, without further description, by Livingston and 

 Laney, as occurring in the contact-metamorphic copper deposits in 

 limestone in the Seven Devils district. 89 



CUSTER COUNTY 



A few small specimens from the Basin group, a prospect on the 

 Custer County side of the divide at the head of Trail Creek in the 

 Hailey quadrangle, mentioned under garnet, prehnite, etc., contain 

 scaly aggregates of minute greenish-brown folia associated with 

 garnet and epidote. Under the microscope this material appears as 

 brown fibrous shreds which are nonpleochroic and have a mean 

 refractive index of about 1.65. This mineral has the appearance of 

 chalcodite which is an altered variety of the chlorite stilpnomelane. 



In specimens of mordenite and other zeolites from near Challis, 

 in Custer County, which have been described in a previous paper 90 

 and which have been mentioned above under mordenite and heulan- 

 dite, about one-fourth of the total number of pebble-like fillings in 

 an amygdaloid rock consist of a soft brownish-green chlorite. The 

 other amygdules are filled with mordenite and heulandite. This 

 chlorite does not occur in the same vesicles with any of the zeolitic 

 minerals, but it is sparingly distributed in the silicified portions of 

 the inclosing rock adjacent to some of the large zeolite-filled cavities. 



The small chlorite fillings which reach an extreme diameter of 

 about 7 mm. are easily freed from the surrounding matrix and, when 

 crushed, yield a homogeneous sample for analysis. Under the 

 microscope the chlorite is seen to consist of yellow-green irregular 

 folia. These are biaxial and optically negative ( — ) with 2V around 

 15°. The indices of refraction, which are high for a chlorite, are 

 u = 1.615, /3= 1.637, 7=1.638, birefringence y- a = 0.023. The min- 

 eral is strongly pleochroic with X and Y olive-green and Z brownish 

 yellow. This material was analyzed, yielding the following results 

 and ratios: 



" D. C. Livingston and F. B. Laney. Idaho Bur. Mines and Oeol., Bull. 1, p. 62, 1920. 

 »' See under mordenite. 



