Decomposition of Iron Pyrites. 1G7 



ed by the luind-specimcn. It will be seen that these vary wide- 

 ly, fi-om a barely visible iridescence, to efflorescence, to a thick 

 inci'ustation by iron-ochre, and to comj)lete hepatic alteration. 

 Various conditions, however, have of course occurred in the 

 vein or rock-matrix once occupied by the specimen ; in many 

 cases it has been completely enveloi)ed and thus protected from 

 oxidation, however perishable ; in others, though stable, the crys- 

 tals of })yi-ites have probably received stains or deposited crusts 

 of iron-oxide, derived from the decomposition of associated min- 

 erals or of their matrix ; in other instances, the specimens have 

 been exposed directly to the weathering influence of the atmo- 

 s])here. The conditions of oxidation have therefore been too 

 vai-iable to permit entire reliance on the present indications of 

 decomposition, as a measure of stability. 



A. Pyrrhotite. 



A few interesting specimens of this mineral, incidentally col- 

 lected, were included in the investigation, and will be first de- 

 scj-ibed. 



No. 1. Pyrrhotite. Ore Knob, Ashe County, North Carolina. 

 Massive, with iridescent, dark bronze-colored surface, and very 

 irregular fracture. Some division-planes run aj^proximately par- 

 allel, and are more or less wavy. On fresh fracture, an indis- 

 tinct fibration and glistening light reddish gray surface, with 

 slight bronze tinge, which deepens in shade on short exposure. 

 Apparently homogeneous and pure, except from a few small en- 

 closed granules of golden yellow chalcopyrite. Decomposition : 

 abundant iridescent or dark reddish brown films over the nu- 

 merous intersecting surfaces of fracture. Under the micro- 

 scope, a beautiful and general iridescence is seen over the fresh 

 and very uneven fracture, but no visible impurity. 



No. 2. Pyrrhotite. Schneeberg, Tyrol. A drusy crust, upon 

 a fine grained veinstone which is a mixture of pyrrhotite and 

 gray quartz. Brilliant reddish bronze hexagonal plates, with 

 edges deeply striated. Strongly magnetic. On fresh fracture, 

 reddish black, uneven, and glistening. Soft to the knife, with 

 dull brownish black streak. Decomposition : the surface of 

 fracture becomes yellowish and reddish on exposure ; the sur- 

 face of the plates is largely covered by tarnish, sometimes dark 

 blue and iridescent, while the interstices of the surrounding 



