2VZ Decomposition of Iron Pyrites. 



Penn,, No. 53. The constant association of chalcopyrite with 

 the octahedra of pyrite at Weehawken, N. J., No. 70, and its 

 occasional enclosure within the latter, have led me to suspect at 

 least its partial connection with the rapid iridescence and ready 

 decay of this pyrite. The arseno-pyrite, indicated by the detec- 

 tion of arsenic in the analysis, already given, of the pyrite from 

 Marsden's Diggings, Galena, 111., No. 3, at tlie head of the 

 Table, may very likely have its influence on the quick oxidation 

 and efflorescence of the specimens from that vicinity, Nos. 1, 2 

 and 3. So too the millerite, attached to the pyrite from the 

 Gap Mine, Penn., No. 108, probably shows that the remarkable 

 instability of the latter mineral may be entirely due to a content 

 of nickel-sulphide. 



3. Relationship of density to crystalline form. The examin- 

 ation of the series of pyrite-specimens afforded an opportunity 

 to obtain some definite information on this question, mooted by 

 Malaguti and Durocher, so far as regards pyrite. Their views, 

 inclining to the possibility of such a connection, were strongly 

 opposed by V. Zepharovich^ whose examination, however, ap- 

 pears insufficient, on account of the small number of specimens 

 examined, ten, and the method employed in determining the 

 specific gravity, on crystals unbroken instead of crushed to 

 coarse powder. The figures reported, 5.002 to 5.028, show his 

 crystals to have been pure, but those of Malaguti and Durocher, 

 already cited, 4.402 to 4.973, certainly indicate great impurity. 

 In the sixth column of my table is presented a statement of the 

 carefully identified crystalline forms in each specimen of the 

 series. On the whole, a consideration of these results inclines 

 me to agree in the main with Kenngott's view of the absence of 

 connection between crystalline form and density in pyrite — with 

 the important exception, however, that well defined pyritohedra, 

 with deeply striated faces, invariably possess the highest density, 

 with the yellow color and other properties of the purest forms of 

 pyrite. There also appears some ground to believe that, in the 

 pyrite-crystals of any particular locality, the octahedra are like- 

 ly to be less rich in pyrite and lower in density than the cubes. 



^Kenngott's Min. Notizen, No. 11, (5te Folge), Sitzb. K. Akad. Wiss., 

 Wien, (1853), XI, 392. 



