THE MINERALS OF IDAHO 323 



Arkansaw and Helena mines, the feldspars of the altered granite 

 have a decidedly pinkish tinge due to the alteration and develop- 

 ment in them of a pink mineral, probably the pink variety of zoisite, 

 thulite. In some specimens the pink mineral is abundant, especially 

 so in veinlets which seem to have developed along fractures in the 

 granite. While no chemical work has been done on this mineral its 

 optical properties indicate that it is thulite. All of the feldspars, 

 whether pink or not, contain much zoisite. 



In the Red Ledge mine — a low-grade disseminated deposit in por- 

 phyritic and fragmental andesite — zoisite is rather widely distributed 

 throughout the rock, and in some places, notably in the highly miner- 

 alized rock forming the ore shoot in the main tunnel, it is abundant. 

 It is usually formed as irregular masses of interlocking crystals or 

 blades, and was apparently developed from both the feldspathic 

 material in the groundmass and the feldspar phenocrysts. Much 

 of it occurs in the vicinity of more or less completely altered pheno- 

 cvsts. 



SHOSHONE COUNTY 



Several specimens from a locality southeast of and across a 

 branch of the Clearwater River from Goat Peak near the southeast 

 corner of the Avery quadrangle contain zoisite in prismatic crystals 

 in a quartz gangue. The specimens, which are from a surface out- 

 crop, are somewhat iron stained. The zoisite is abundant as rude 

 prisms up to 1 cm. thick by 2 cm. long. Adjacent to the walls of the 

 veins the quartz contains some biotite and garnet. Where un- 

 stained by limonite the zoisite crystals are dirty white in color. 

 They possess the usual cleavage parallel to 6(010), are untermi- 

 nated, and are bounded simply by the unit prism m(110) which is 

 somewhat rounded and striated. Under the microscope the mineral 

 is filled with small inclusions, aligned in a vertical direction, but is 

 otherwise transparent and colorless. It is biaxial positive with 2V 

 medium small, dispersion strong, r<v. The optical orientation is 

 X = c, Y = 6, Z = et, and the refractive indices are a = 1.700, /3 = 1.703, 

 7-1.718, all ±0.003. 



A sample was separated from quartz by methylene iodide, the 

 staining limonite removed by treatment with hydrochloric acid, and 

 a few grains of garnet removed with an electromagnet. This was 

 analyzed with the following results : 



Analysis of zoisite from Avery quadrangle 



(E. V. Shannon, analyst) 



Percent 



Silica (Si0 2 ) --- 39.20 



Alumina (A1 2 3 ) 33. 22 



Ferric iron (Fe 2 03) 2. 14 



Lime (CaO) 23.90 



Barium oxide (BaO) Trace. 



