208 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



material. The crystalline structure is what identifies the mineral as 

 goethite rather than limonite and this, in most cases, is best de- 

 termined by observing its double refraction under the microscope. 



BLAINE COUNTY 



Specimens of iron oxide from the Narrow Gauge shaft are cellular 

 sponges made up of finely fibrous metacolloidal goethite. 



CUSTER COUNTY 



A lot of material sent in to the National Museum for identification 

 by Mr. J. W. Cooper from Hill City, Custer County, consists of 

 small botryoidal goethite, with a finely fibrous structure, on fractures. 



LEMHI COUNTY 



Ordinary small-bo tryoidal radial fibrous dark-brown masses of 

 goethite occur in cavities in quartz in specimens from the Big Chief 

 claim, Blackbird district. 



SHOSHONE COUNTY 



Botryoidal goethite is probably found near the surface in the 

 oxidized ore of a majority of the Coeur d'Alene mines. It seems 

 especially to occur at or very near the surface and small black coatings 

 are universally present. Fine specimens occur in a prospect in the 

 basin at the head of the east fork of Milo Creek above Wardner and 

 in various shallow openings about the flanks of Snow Patch (locally 

 called Kellogg) Peak, including one prospect tunnel on the southeast 

 side of the shoulder immediately below the summit of the peak. 

 Specimens occur in the outcrop of the Caledonia vein and under- 

 ground in this mine at shallow depth. Specimens of this mineral 

 have also been examined from the Palisade claim on Pine Creek. 



LIMONITE (259) 



Hydrated iron oxide, 2Fe 2 03.3H 2 0. Amorphous 



Limonite, the hydrated iron oxide which is probably the amorphous 

 equivalent of goethite, is one of the commonest of minerals formed 

 under the influence of surface weathering. It results from the de- 

 composition and oxidization of iron-bearing minerals, not only the 

 sulphides and carbonates of veins but also from ferrugineous silicates 

 such as hornblende, pyroxenes, chlorites, iron-bearing micas, etc. 

 Common iron rust is limonite and as rust brown stains this mineral 

 is present everywhere. In veins the limonite results from the oxida- 

 tion of iron-bearing sulphides, as pyrite, pyrrhotite, marcasite, chal- 

 copyrite, arsenopyrite, and bornite; from the alteration of iron- 

 bearing carbonates, as siderite, ankerite, and ferriferous calcite, 

 and from the oxidization and hydration of the oxides, magentite, and 



