Decomposition of Iron Pyrites. 391 



Scotland, etc.," especially when nodules of the mineral are en- 

 veloped in clay. 68 



Marcasite is also subject to decomposition into hyd rated iron- 

 oxide, like pyrite, especially when protected from the atmos- 

 phere, as by envelopment in clay, etc. 



Thus psendomorphs in limonite after marcasite have been 

 found in Bohemia, Hungary, the Orkney Islands, Scotland, etc. 60 



Senft has suggested that such psendomorphs of iron -ochre, 

 after pyrite and marcasite, may have been caused by a vitrioles- 

 cence affected by solutions of alkaline carbonates, resulting in 

 the alteration of the pyrites, atom by atom, into iron carbonate, 

 afterwards converted by higher oxidation and loss of carbonic 

 acid into iron-oxide. 



One constituent of these limonite nodules, after marcasite or 

 mixed pyrites, possesses great interest, viz., native iron, which 

 has been detected in them, as small grains or scales, near Chot- 

 zen, Bohemia; near Muhlhausen ; near Por crush, Scotland, and 

 in California. 61 



Paragexesis of the Irox Pyrites. 



During our consideration of the variations in density of the 

 three forms of iron pyrites, reference has been made to the in- 

 fluence of included cavities, the work of oxidation, etc. But 

 beyond the several agencies thus far alluded to, there is evidence 

 of some other of universal prevalence, to account for these con- 

 stant and wide variations. We need to go farther than the 

 opinion generally accepted, and which has been thus expressed 

 by v. Zepharovich, in regard to his own examination of the 

 density of pyrite crystals, that "only when the intermixture of 

 foreign particles occurs, or the pyrite crystals have suffered al- 

 teration, are the specific gravity figures modified." 



The additional influence to which I refer is that exerted by 

 the several natural iron sulphides upon each other, not merely 

 by intermixtures and intercrystallizations, but either by iso- 



5 > Blum, Pseudomorphosen (1843), 197—199, and Nacktrag (1803), 185. 



59 Blum, op. cit., Nachtrag (1847), 107—112, 



60 Roth, op. cit., 105. 



61 Idem, 236. 



