132 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



WASHINGTON COUNTY 



In Washington County many of the large hematite deposits in the 

 vicinity of Iron Mountain have been found to pass at shallow depth 

 into massive bodies of pyrite. On the Segwine claims a 5-meter (15- 

 foot) lens of pyrite is exposed and in the Little Gem claim there is a 

 vein of pyrite 2 meters (7 feet) wide and a zone of garnet rock thickly 

 impregnated with pyrite for a width of 30 meters (90 feet). 13 



In the Mineral district pyrite occurs commonly in gold and silver 

 ores associated with galena, sphalerite, tetrahedrite, etc. An unusual 

 variety has been reported from this district containing 10.9 per cent 

 of manganese 14 and a little silver. 



COBALTITE (89) 

 Cobalt sulpharsenide, CoS 2 .CoAs 2 . Isometric, pyritohedral. 



LEMHI COUNTY 



Cobaltite is abundant in many cobalt mines and prospects in the 

 Blackbird district being, so far as the specimens at hand indicate, the 

 only important cobalt ore mineral of this field. It occurs as reddish 

 gray grains, crystals, and granular masses. 



Very fine-grained reddish patches in schist from the Gray Eagle 

 property are unaltered in the closed tube over the Bunsen burner but 

 at blast temperature give characteristic rings of arsenic sulphide and 

 arsenic, and are thus cobaltite. Ore from the Tom Jefferson claim 

 contains minute reddish-gray crystals of cobaltite with chalco- 

 pyrite making up a sulphide band 1 cm. wide in schist. Crystals 

 from this specimen were measured and found to be isometric 

 pyritohedral in crystallization with faces of the cube, octahe- 

 dron, and pyritohedron. Specimens from the Brooklyn claim 

 also show crystals of cobaltite up to 2 mm. in diameter associated 

 with chalcopyrite. These show rather perfect cubic cleavage. It 

 dissolves with avidity in concentrated nitric acid with separation of 

 sulphur and yields a pink solution which becomes green when a little 

 hydrochloric acid is added. It is unaltered when heated in a closed 

 tube over a Bunsen burner and gives arsenical fumes when heated 

 before the blowpipe on charcoal. It contains no iron and gives no 

 nickel reaction with dimethylglyoxime. The crystals have the forms 

 shown in Figure 13, the cube modified by octahedron and pyritohe- 

 dron. The measurements which establish their isometric form are 

 given in the following table : 



13 Robert N. Bell. 20th Ann. Rept. Mining Industry of Idaho, pp. 107-109, Boise, 1918. 



14 Lang. Private communication in supplement sixth edition of Dana's System of Mineralogy, p. 1045. 



