366 Decomposition of Iron Pyrites. 



This process is rapid and implies apparently, in most cases, 

 an abundant supply of moist air and rapid oxidation, but pro- 

 tection from the flow of meteoric waters. 



Marcasite is distinguished by its ready subjection to this mode 

 of weathering, which however often attacks nodular and con- 

 cretionary pyrites of mixed constitution and some varieties of 

 pyrrhotite. 



For the clear understanding of the causes of these various 

 phenomena of decomposition, it will be desirable to consider : 

 first, the relationship of composition to density and other phy- 

 sical properties in the artificial and natural sulphides of iron, 

 the origin and association of the latter, and their common 

 modes of decomposition ; and, secondly, the variation of decom- 

 position in pyrite, its probable cause, and the modes of discrimi- 

 nation between stable and unstable varieties of the mineral. 



Part I. 

 The Sulphides of Irox. 



There are three iron sulphides theoretically recognized in 

 chemistry, but of even these the existence of one cannot yet be 

 regarded as established. They may be artificially prepared, in 

 conditions of uncertain purity, by various methods. 



Iron protosulphide, or ferrous sulphide, Fe S. When pre- 

 pared by the process of G-ahn, stirring a white-hot rod of iron 

 in molten sulphur, it forms "a yellowish crystalline mass, hav- 

 ing a metallic lustre, and sometimes crystallizing in hexagonal 

 prisms," 1 with specific gravity of .4.69. But when prepared by 

 throwing a mixture of three parts of iron filings and two parts 

 of sulphur into a red-hot Hessian crucible, " it is thus obtained 

 as a black porous mass, which at a higher temperature fuses, 

 solidifying to a greyish yellow, crystalline metallic mass of spe- 

 cific gravity 4.79 ." a 



Rammelsberg, 3 by heating octahedral pyrite from Elba in 

 hydrogen, prepared it of a specific gravity 4.694 ; and Rose, 4 by 



1 Roscoe and Schorlemmer, Treat, on Chem. (1880), II, 118. 



2 Frankland and Japp, Inorg. Chem. (1884), 766. 



3 Pogg. Ann. (1864), CXXI, 337. 



4 Idem (1849), LXXIV, 301. 



