THE MINERALS OF IDAHO 145 



MARCASITE (96) 



Iron disulphidc, FeS-2 Orthorhombic 



Marcasite, commonly called white iron pyrite, is identical with 

 pyrite in composition but differs in crystallization. When it is 

 massive it is difficult to distinguish from pyrite, although it is usually 

 paler in color and is more readily decomposed by weathering. It is 

 much rarer than pyrite. 



BLAINE COUNTY 



Marcasite is reported to have been mined in the Wood River 

 district as an ore of gold. 17 



OWYHEE COUNTY 



Marcasite occurs in a number of silver mines in the Silver City 

 district. In the Chautauqua tunnel and in the Trade Dollar and 

 De Lamar mines it occurs in dendritic forms in quartz, and in speci- 

 mens from the Garfield tunnel it is abundant in cla} 7 . 18 



LEMHI COUNTY 



Beautifully spherical nodules of marcasite up to 1}4 inches (4 cm.) 

 in diameter occur in the Tertiary beds a mile south of Salmon 

 These are usually dull on the outside and are aggregated into fan- 

 tastic forms like those of ordinary concretions. When broken open 

 they are found to consist of very pale greenish granular marcasite 

 having an indistinctly radiated structure. The inclosing rock is 

 loosely cemented clay sandstone. 19 



SHOSHONE COUNTY 



In the Gold Hunter mine at Mullan cavities in the ore sometimes 

 contain bright spherical or botryoidal masses of iron sulphide, 

 already mentioned under pyrite. These masses are bright and 

 have a pale yellowish color and brassy luster. The exteriors are 

 usually smooth and when broken open the masses are seen to be 

 radiated columnar in structure. It is quite probable that some of 

 these masses are marcasite. 



SYLVANITE (104) 



Gold silver telluride (Au, Ag) Te 2 . Monoclinic. 



Despite the large number of gold veins in Idaho, tellurides have 

 been met but rarely and in small amount. Sylvanite is the name 

 most used by miners and prospectors for ores in which the gold is 

 not visible to the naked eye and reports of the occurrence of sylvanite 

 or of tellurides must be discounted for this reason. So far as known 

 no accurate mineralogical identification of any gold telluride from 



" U. S. Qeol. Survey, BuH. 624, p. 119, 1917. 



'« Waldemar Lindgren. U. S. Geol. Survey, 20th. Ann. Report, pt. 3, 1900. 



» J. B. Umpleby. U. S. Qeol. Survey, Bull. 528, p. 78, 1913. 



