Decouijjuaitiun of Iron Pyrites. 137 



more liable to decomi)osition tbiiii i)yrite, though in the same 

 manner, and most frequently changes to sulphate of iron. In 

 both minerals this has been ascribed to a mixture of Fe S ; but 

 the above anal3"ses all show a surplus of sulphur, which, though 

 small, would rather indicate a mixture of free sulphur as the 

 cause. It, however, has been observed in pure sulphuret of 

 iron, and seems rather to depend on some peculiarity in the 

 state of aggregation." More recently a similar view has been 

 expressed by J. P. Kimball,^ to explain the difference in stabil- 

 ity of the two minerals. " At ordinary temperatures, underex- 

 posure to the atmosphere, bi-sulphide of iron in the form of 

 white pyrites or marcasite readily decomposes, but far more 

 slowly when in the form of yellow jiyrites or pyrite. Such a 

 difference as to facility of oxidation appears to be due to mole- 

 cular differences arising from crystalline structure. Marcasite 

 occurs in several states of crystallization and aggregation favor- 

 able to oxidizing influences, as compared with the usually more 

 compact mode of occurrence of pyrite. It is in the more readily 

 decomposable form of marcasite that bi-sulphide of iron general- 

 ly occurs in bituminous coals and lignite, while in anthracite it 

 is generally, if not always, present in the form of its more stable 

 species." It is evidently in this direction that we must search 

 for the exact solution of the problem. 



C. Chemical Analysis of Pyrites. 



On account of the doubt still prevailing in regard to the com- 

 monly accepted cause of the tendency to decomposition, viz., the 

 possible presence of iron protosulphide in decomposing pyrites, 

 there seemed to me to be evident need of additional chemical 

 analyses, made with the utmost care possible with modern facil- 

 ities, on crystals selected for probable purity ; in these I have 

 availed myself of the assistance of Dr. J. B. Mackintosh. For 

 this purpose, the finely crystallized spear-headed marcasite, 

 from the ashy gray chalk of Folkestone, England, was first 

 chosen, this matrix being unlikely to supply other metallic 

 bases or arsenic as impurities in the mineral. The following 

 mode of analysis was adopted. A carefully weighed amount of 



1 Trans. Am. Inst. Min. Eng., (1879-80), VIII, 203 and 215-316. 



