Determination of Pyrite and Marcasite. 



389 



These are the figures for Roxbury pyrite and Joplin mar- 

 casite. We have seen above that these results are affected by 

 the presence of arsenic in the Roxbury pyrite. In fig. 6 are 

 graphically shown the Stokes constants (x-a), for the purest 

 natural pyrite and marcasite (Curve 1, 1) and also for the 

 synthetic minerals (Curve 2, 2). It must be admitted that the 



Fig. 5. 



.-o 60 



J2 50 



=5 

 V) 



O 



^0 



30 

















































































y 







































/ 



/ 



































i 



s 



' 















































































































































J--"? 



) 





















































































































































































10 20 30 40 50 60 



Percentage of pyrite 



70 



80 



90 



IQ0 



Fig. 5. Curve showing sulphur oxidized in pyrite -marcasite mixtures by 

 a standard ferric solution. Roxbury pyrite. Joplin marcasite. 



value 10*3 for synthetic marcasite is open to the suspicion 

 that the products may have contained a little pyrite.* Fig. 7 

 shows the hyperbolas calculated from these results, the assump- 

 tion being made that the constants are exactly additive in 

 their mixtures, while the points plotted are, as in fig. 5, cal- 

 culated from the experimental values as given in fig. 4. 



Fig. 8 shows Stokes' published values for p plotted against 

 y. The plotted curve is again a true hyperbola calculated 

 from the values of x which fall on the line in Hig. 1. The 

 minimum in his curve which Stokes obtained at 10 per cent 

 pyrite seems therefore to have no real existence. 



Summary. 

 1. The Stokes method for determining pyrite and mar 

 casite, alone or in mixtures, depends on the estimation of the 

 *This Journal (4), xxxviii, 411, 1914. 



