198 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



The source of this mineral in these fine sands of the Snake River 

 is not certain. It is noteworthy, however, that the thin sections of 

 the diabasic rock, which occurred abundantly in sand from Minidoka, 

 contained numerous scattered tabular crystals of ilmenite. 



SPINEL (234) 



Magnesium-iron aluminate, (Mg,Fe)O.Al 2 3 . Isometric. 



Spinel has been mentioned as occurring at a few places in Idaho 

 but nowhere in large crystals. 



ADAMS COUNTY 



Spinel is listed, without description, by Livingston and Laney e 

 as occurring in the contact copper deposits of the Seven Devils 

 district. 



CUSTER COUNTY 



Small grains of spinel occurring with chondrodite in a chondrodite 

 limestone from the head of Wildhorse Canyon in the Hailey quad- 

 rangle have been described under chondrodite. 



MAGNETITE (237) 

 MAGNETIC IRON 



Ferrous-ferric oxide, FeO.Fe 2 3 . Isometric. 



BLAINE COUNTY 



Magnetite occurs in small amount in the Wood River district as a 

 contact mineral in limestone. 



CUSTER COUNTY 



Magnetite, as is to be expected, is abundant in the vicinity of the 

 contact-metamorphic copper deposits of the Alder Creek district. 

 It is abundantly developed in the Iron and Quarry tunnels and is 

 found locally throughout the deposits. It is characteristically fine- 

 grained and associated with specularite or an equal amount of gar- 

 net. It is not plentiful in association with chalcopyrite. On the 

 ridge northwest of the Copper Bullion tunnel it occurs as veinlets of 

 coarse crystals cutting garnet rock. Here the garnet contains cores 

 of magnetite. Common assays of the magnetite rock show 50 per 

 cent iron, 1 per cent of copper, 0.002 ounce gold, and 0.57 ounce 

 of silver a ton. 7 



LEMHI COUNTY 



Magnetite in moderate amounts is widespread in occurrence. In 

 the Carmen Creek district it is found in the ores included in the 

 vein quartz where the adjacent wall rock contains actinolite and 



» Idaho Bur. Mines and Geol., Bull. 1, p. 62, 1920. 



» J. B. Umpleby. U. S. Geol. Survey, Piof. Paper 97, pp. 53-54, 1917. 



