418 H. N. Stokes — Pyrite and Marcasite. 



"With these data, a curve is drawn on cross section-paper from 

 which it is possible, after finding the oxidation coefficient of 

 the mixture, to read off the relative proportions of pyrite and 

 marcasite to within 1 per cent when the mixture is quite rich 

 in pyrite and to 2 or 3 per cent when it consists mainly 

 of marcasite. The existence of a well marked minimum in 

 mixtures with 10 per cent pyrite is noteworthy. I have 

 detected similar minima and maxima in curves given by mix- 

 tures of pyrite with other minerals. The uncertainty of 

 duplicate values for mixtures with less than 25 per cent 

 pyrite, caused by this minimum, is obviated by mixing the 

 specimen with a known quantity of pyrite and determining 

 the oxidation coefficient of the new mixture. If x represents 

 the percentage of pyrite in the original sample, a mixture of 90 

 parts of this with 10 parts pyrite will give 



Q'Sx + 10 = per cent pyrite in new mixture, 



whence a?, composition of the original material, is easily deduced. 

 Here, as before, the results express simply the percentage com- 

 position of the iron disulphide, indifferent substances being 

 without effect. 



In regard to the influence of impurities, it may be stated 

 that substances which do not contribute iron to, or effect 

 reduction of, the ferric salt are without influence. Pyrrhotite, 

 limonite, siderite, and other soluble iron compounds and zinc- 

 blende or galena in small amounts may be extracted by heating 

 with dilute hydrochloric acid; nickel and cobalt and other iron 

 free sulphides not extractable by acid give abnormal results. 

 For details as to the behavior of impurities reference must be 

 made to U. S. Geological Survey Bulletin No. 186. In this 

 connection, it is interesting to note that in a pyrite carrying 3 

 per cent copper it is possible to ascertain whether the copper 

 be present as chalcopyrite, or as chalcocite or bornite, a deter- 

 mination which could scarcely be made by the usual analytical 

 methods. The values of j?* for a 3 per cent mixture were found 

 to be : 



Pyrite-chalcopyrite 62 *7 



Pyrite-chalcocite 15 # 9 



Pyrite-bornite .76*4 



The difference is many times the probable error of a determi- 

 nation. 



Small amounts of chalcopyrite mixed with pyrite or marca- 

 site may be readily detected by exposing the sample to bromine 



* p is here obtained as before by substituting the permanganate values in the 

 above equation, but in this case it does not represent strictly the percentage of 

 sulphur oxidized. 



