THE MINERALS OF IDAHO 207 



CUSTER COUNTY 



In the copper deposits of the Alder Creek (Mackay) district, pyro- 

 lusite occurs as dendrites on fracture surfaces m the limestone, and 

 as black, sooty material mixed with iron oxides in the oxidized ore. 17 

 In northwestern Custer County, dendritic pyrolusite is not uncom- 

 mon along fractures in the lead-silver deposits. 18 



LEMHI COUNTY 



The pyrolusite found in Lemhi County is worthy of special men- 

 tion because of its beautiful dendritic forms. These, which resemble 

 frost figures or branching vegetation, are abundant in many places 

 along fractures and bedding planes in limestone adjacent to the lead- 

 silver deposits. Manganese oxide is coextensive with iron oxide. In 

 many places in the lead silver deposits it is very conspicuous. 19 



SHOSHONE COUNTY 



Manganese oxide, probably including some pyrolusite, is wide- 

 spread in occurrence, mixed with iron oxides and cerusite, in the 

 upper portions of the lead-silver veins of the Coeur d'Alene district, 

 as further discussed under psilomelane. A few occurrences of well- 

 characterized pyrolusite have been noted. In a rusty and much 

 brecciated quartz vein encountered by the lower tunnel of the north 

 Bunker Hill at the west side of Milo Creek at the east base of Hay- 

 stack Peak at Wardner, steel gray crystalline rosettes of pyrolusite 

 were found in 1915-16. On the 800-foot level of the Hercules mine 

 at Burke an open crack along a fissure in quartzite was lined with a 

 crust of crystals of calcite upon which were deposited mossy masses 

 of minute pyrolusite crystals. Fibrous striated blades and masses 

 of p3 T rolusite, secondary after manganiferous siderite, occur with 

 limonite in quartz from the Black Diamond claims in the Pine 

 Creek district. 



GOETHITE (257) 



Hydrous iron oxide, Fe 2 03.H 2 0. Orthorhombic. 



Goethite is a crystalline form of ferric hydroxide having the same 

 composition as limonite, with which it is commonly associated. 

 Limonite is amorphous and usually holds some excess water. This 

 amorphous ferric hydroxide tends to go over to the crystalline form 

 becoming goethite, having a very finely fibrous form and black 

 botryoidal lustrous surfaces. While goethite is known to occur in 

 some localities elsewhere as distinct prismatic crystals, such crystals 

 have not, thus far, been reported from Idaho and the occurrences 

 mentioned below are all of the metacolloidal finely fibrous botryoidal 



" J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 94. 

 >« J. B. Umpleby. U. S. Geol. Survey, Bull. 539, p. 53, 1913. 

 >s J. B. Umpleby. U. S. Geol. Survey, Bull. 528, p. 78, 1913. 



