THE MINERALS OF IDAHO 455 



Optically it was found that this mineral is biaxial negative with 2V 

 large, dispersion r<v, large. Color green and nonpleochroic. Ex- 

 tinction parallel. There is a good cleavage parallel to the obtuse 

 bisectrix or parallel to (001). The indices measured are a = 1.727, 

 /3 = 1.778. The higher index was higher than the highest oil available 

 at the time the measurements were made. 



CUSTER COUNTY 



In the Alder Creek district (Mackay) brochantite is almost as abun- 

 dant as malachite with which it is often intergrown. In a few speci- 

 mens the brochantite is intergrown with azurite affording a beautiful 

 blending of blue and green colors. Rarely it is crystallized in short 

 stout rhombic prisms which are vertically striated. In places in the 

 300-foot Empire tunnel it forms the central portion of narrow veins of 

 azurite in limestone. 55 



Brochantite has also been noted as crusts of minute emerald green 

 crystals lining cracks in iron-stained rock in specimens from the 

 Parallel tunnel, Parallel claim, Reed and Davidson mine, in the 

 Copper Basin district, at the head of the east fork of Big Lost River. 



LINARITE (741) 



Basic sulphate of lead and copper, Monoclinic. 



PbO.CuO.S0 3 .H 2 0. 



The rare blue monoclinic double sulphate of copper and lead occurs 

 occasionally in oxidized ores where lead, copper, and sulphuric acid 

 have been available during oxidation. It usually forms small drusy 

 or columnar masses closely resembling azurite. Compared with 

 azurite, however, the color is a brighter blue and the luster of the 

 linarite is more brilliant than that of azurite. The following localities 

 in Idaho have been noted: 



CUSTER COUNTY 



A specimen from the Pacific mine, Bay Horse district, Custer 

 County, consists of white quartz containing disseminated galena and 

 tetrahedrite partly oxidized to waxy compact bindheimite and fine 

 blue linarite. Space for the formation of well-bounded linarite 

 crystals has been lacking but a few euhedral individuals occur in 

 minute vugs. Only one crystal could be secured from the small 

 specimen for measurement. The development assumed is shown in 

 Figure 159. The forms and angles measured are as follows: 



" J. B. Umpleby. U. S. Qool., Survey, Prof. Paper 97, p. 50, 1917. 



