] 40 Decomposition of Iron Pyrites. 



oxidation, but they never display an efflorescence of iron sul- 

 phate. The next material, selected for analysis, was the fibrous 

 white and brilliant pyrite, associated with galenite and blende, 

 at Marsden's Diggings, near Galena, Illinois, and partially dis- 

 colored by an efflorescence of iron sulphate. A lump, freed 

 from the efflorescence and weighing about 3 grams, was dis- 

 solved in the usual way, and the following results obtained. 



Analysis of fibrous pyrite, from Marsde^i^s Diggings, III. 

 Sp. Gr. = 5.015. 



Per Cent. Theoretical Constitution. 



Iron 46.636 46.667 



Lead .188 



Silica .110 



Arsenic .056 



100. 100. 



An absolutely exact determination of sulphur is impossible by 

 any known gravimetric method ; this is partly caused by the de- 

 composition of a sniall part of the precipitate obtained of bar- 

 ium sulphate adhering to the filter, by the deoxidizing action of 

 the carbon, on ignition of the paper. In determinations of this 

 kind, a similar loss almost invariably occurs, amounting to over 

 one per cent, of the total quantity of sulphur, the ignited pre- 

 ci})itate being apt to assume a yellow color, it may be from the 

 formation of barium peroxide. It therefore seems fair to pre- 

 sume that the loss in the above analysis consists chiefly of sul- 

 phur. The figures thus imply a close correspondence to those 

 demanded by theory, if the material was pure. The slight im- 

 purity, indicated by traces of lead and arsenic, is merely that 

 which might be expected in a mineral which is constantly asso- 

 ciated with galenite in the locality stated. We may therefore 

 conclude that decomposing pyrite contains no proportion of iron 

 proto-sulphide, and that the cause of the decomposition must be 

 sought in some other direction. The analyses of both these 

 minerals therefore, marcasite and i)yrite, prove that difference 



' 53 617, before purification of the precipitate of barium sulphate. 



