214 Decompositio7i of Iron Pyrites. 



to the occurrence and influence of the other two pyrites. It is 

 highly probable that, in the manufacture of alum, copperas and 

 sometimes that of sulphuric acid, its rapidity, and the volume 

 and cost of the product, may largely depend upon the degree of 

 ready oxidation of the pyrites employed. The comparatively 

 refractory character of certain forms of these minerals, such as 

 the auriferous veinstones of the far West, may yet be an impor- 

 tant factor, usually hitherto disregarded^ in estimates of their 

 future commercial value. The freedom with which a pyritous 

 ore submits to calcination is obviously an element to be con- 

 sidered in the selection of beds of mineral for these purposes. 

 In his discussion of *^the characters which best adapt pyrites 

 for the use of acid manufacturers," S. Gr. Williams includes 

 ** readiness to part with the contained sulphur, in which differ- 

 ent lots of pyrites show^ considerable differences."^ These he at- 

 tributes to the physical condition of the mineral, to differences 

 of fusibility caused by certain included minerals, and to the re- 

 tention of sulphur by other included sulphur-compounds, like 

 those of copper. To these causes, it is apparent, should be ad- 

 ded the presence and influence of pyrrhotite and marcasite, 

 whose greater inclination to decomposition must materially 

 modify the refractory character of a pyritous ore. The contrary 

 property, resistance to oxidation and disintegration, is equally 

 advantageous in connection with the storing of mined pyrit- 

 iferous coal and the choice of pyritiferous building-stones and 

 roofing slates. 



A. Pyrites ik Coal. 



An important question has long awaited solution, as to the 

 cause and prevention of that slow oxidation Avhich goes on in 

 masses of mined coal, commonly producing a process of crumb- 

 ling or ^'slacking", which may greatly diminish its commercial 

 value, and sometimes endangers the coal by a sensible elevation of 

 temperature, reaching even to spontaneous ignition. The 

 following reference^ to this has been made by J. P. Kimball : 

 *'Among the ordinary circumstances favorable to the weathering 



1 W. Martin, on Pyrites, Williams, Min. Resources of the U. S., (1883- 

 84), 880. 

 « Applied Geology, (1886), 300. 

 8 Trans. Am. Inst. Min. Eng., (1879-80), VIII, 215-217. 



