Decomposition of Iron Pyrites. 373 



erai an association, in some form of replacement or combina- 

 tion, of iron protosulphide, Fe S, having a specific gravity of 

 about 4.4 or 4.5, as before estimated,— with iron disnlphide, 

 Ke S 2 , of specific gravity equal to 5.0, the density of the varie- 

 ties of pyrrhotite increasing with the proportion of iron disnl- 

 phide. Nor is it necessary to conceive, it appears to me, that 

 the disnlphide in combination must possess the physical proper- 

 ties of one of its crystallized forms, pyrite or marcasite. It may 

 be decomposable by acids, and this may account for the fact 

 that not pyrite but sulphur is left on digestion of pyrrhotite in 

 hydrochloric acid. 



We may here refer to the ingenious suggestion of L. Bom- 

 bicci, 12 that pyrrhotite may be simply made up of the regular 

 association of two octahedral or isometric elements, *. e., of true 

 pyrite, Fe S 2 and of magnetite, Fe 3 4 , in the proportion of 4- 

 1 ; i. e., 4 Fe S 2 + Fe 8 4 = Fe 7 S 8 0\ Though the absence of 

 oxygen in pyrrhotite is inconsistent with this hypothesis, it has 

 served to suggest that there may yet be found in this mineral 

 some analogous association of molecules of isometric pyrite, 

 Fe S 2 , and of iron protosulphide, Fe S, which, like pyrrhotite' 

 was found by Gabn "sometimes crystallizing in hexagonal 

 prisms." The analyses of varieties of pyrrhotite, beginning 

 with the formula Fe 5 S 6 , have at last reached Fe 16 S 17 , and it 

 seems likely that some variety may soon be found in which the 

 relationship of iron and sulphur will be as 1 to 1. The normal 

 composition of this mineral may then indeed be, as was held by 

 Frankenheim, that of iron protosulphide; and it is a plausible 

 hypothesis that, from natural conditions of paragenesis, more 

 or less pyrite, Fe S 2 , in octahedra, may be generally intermixed 

 through the native mineral in conformity to crystallographic 

 symmetry, and also enclosed in its artificial form, left as a resi- 

 due on the ignition of pyrite. 



Weathering of Pyrrhotite. 

 The extent of the distribution of this mineral and its abun- 

 dance are comparatively so limited that its mode of decomposi- 

 tion appears to have been little studied. 



12 Nuovi studj sulla Poligenesi dei minerali, Mem. Ace. Sci. 1st. Boloo-na 



(1883), 27, 28. 



