392 Decomposition of Iron Pyrites. 



morphous replacements in some cases, or by internal develop- 

 ment through molecular re-arrangement. In order then to 

 obtain a full appreciation of these effects upon the. density, it 

 appears desirable to consider the genetic relationship of the three 

 species, their natural intermixtures and inter-crystallizations, and 

 their isomorphous replacements. 



As to the geognostic distribution of the varieties of pyrites, 

 and an actual succession in their ordinary genetic history, some 

 interesting facts may be recognized. 



First, the development of iron sulphide within the mass of 

 sedimentary deposits, at ordinary temperatures — it may be, chiefly 

 as the very unstable protosulphide — has generally resulted in 

 the first place, 62 in the production of scattered amorphous parti- 

 cles of pyrrhotite ("iron sulphide in minimo" of Stromeyer), 

 with an excess of the metal and a varying but inferior amount of 

 sulphur, compared with the proportions in the disulphide. In 

 the lmignetism of this product we may very probably find an in- 

 dication which has favored both its development and its later 

 concentration, by the electric currents which have always at- 

 tended the consolidation, initial crystallization, chemical reac- 

 tions, and metamorphism within sedimentary deposits. Another 

 result appears to have been the combination of a small part of 

 the sulphur at once in the form of Fe S 2 . the greater part as 

 Fe S, viz. : 



Fe 5 S 6 = 4 Fe S + Fe S 2 , 

 Fe' S 7 = 5 Fe S + Fe S 2 , 

 etc., up to 



Fe 16 S 17 = 15 Fe S + Fe S 2 . 



In reference to the agent and reaction concerned in the devel- 

 opment of natural sulphides, T. S. Hunt 63 has suggested : 



"I have found that the unstable protosulphide which would 

 naturally be first formed, may, under the influence of a persalt 

 of iron, lose one half of its combined iron ; and that from this 

 reaction a stable bisulphide results. This subject of the origin 

 of iron-pyrites is still under investigation." 



62 Knop, N. Jahrb. f. Min. (1873), 521. 

 153 Ckem. and Geol. Essays (1875), 230. 



