136 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



The ratios indicate the formula (Ni, Fe, Co) AsS with the ratios 

 Ni : Fe : Co = 7 : 5 : 4, approximately. 



The crystals can be isolated from the quartz and those which were 

 measured were found to be isometric combinations of cube and 

 octahedron. 



It is evident that the mineral is a member of the pyrite group and 

 a sulpharsenide of nickel, iron, and cobalt. It is thus an inter- 

 mediate 3-component isomorphous mixture of the gersdorffite and 

 cobaltite molecules with a hypothetical iron sulpharsenide not yet 

 recognized as a distinct mineral. Since the gersdorffite molecule is 

 definitely predominant over the others in it, the present mineral 

 may be designated gersdorffite. 



The color of the mineral is steel gray, like arsenopyrite. After ex- 

 posure it dulls with the assumption of a barely perceptible reddish 

 tinge. In the closed tube it gives, like arsenopyrite, copious subli- 

 mates of arsenic sulphide and arsenic. 



Polished surfaces of the ore, examined in reflected light, disclose 

 the angular gersdorffite which has probably replaced the quartz 

 metasomatically. Pyrrhotite occurs in a network of fine seams 

 forming the matrix of angular quartz grains. The only other metallic 

 mineral present is white in color and is unattacked by the usual etch- 

 ing reagents, including nitric acid and acid permanganate. This 

 may be pentlandite. It is in very small amount and bears the same 

 relation to the quartz as the pyrrhotite. 



The gersdorffite is resistant to all reagents, except that with long- 

 etching with acid permanganate a faint zoning becomes visible by 

 the slight darkening of certain zones in the crystals showing, prob- 

 ably, differences in composition. 



The associated pyrrhotite, when separated and purified, contains a 

 mere trace of nickel. 



ARSENOPYRITE (98) 

 Iron sulpharsenide, FeAsS. Orthorhombic 



Arsenopyrite, commonly known as arsenical pyrites, or mispickel, 

 occurs rather widely in Idaho as an accessory mineral in lead, zinc, 

 gold, and antimony veins. While usually not itself of commercial 

 value it sometimes contains notable amounts of gold and, occasion- 

 ally, of silver. The arsenic content complicates the metallurgical 

 treatment of the ores and usually a penalty is assessed against 

 arsenical ores by smelters. Where in large bodies favorably situated 

 this mineral has a prospective value as a source for arsenic which is in 

 demand for the manufacture of poisonous insecticides. No bodies of 

 commercial arsenopyrite have thus far been developed in Idaho. 

 Such occurrences of the mineral as have been brought to the writer's 

 attention are mentioned below. 



