Determination of Py rite and Marcasite. 385 



stated more than once by Baskerville and Stevenson* and 

 others that an acid ferrous solution is not readily oxidized by 

 the oxygen of the air, but we have here in addition the possi- 

 bility of an oxidation of the finely ground sulphide. Since, 

 however, moist sulphide oxidizes so very slowly even when 

 exposed to the direct influence of the air, oxidation should be 

 negligible when the filtration is done in our apparatus. 



4. Accuracy of the method. — The systematic study of this 

 problem has revealed only two considerable sources of error 

 and means for obviating them have been explained. The 

 results in fig. 3 involve errors of one per cent to two per cent. 

 "While this is a large error in ordinary analytical operations, it 

 must be borne in mind that the two minerals are very similar 

 and the method is very delicate. The total difference between 

 the quantities of iron which the two minerals yield to the 

 standard solution is, in terms of a dilute permanganate solu- 

 tion, only 6*1 to 6'4 g., according to the sample of pyrite taken, 

 i. e., only 0*06 g. for every per cent. This amounts to about 

 1-5 drops of the permanganate solution, containing about 0*1 

 mg. of KMn0 4 and equivalent to only about 0*2 mg. of iron 

 dissolved. 



While the results are very constant for the same specimens 

 of pyrite and marcasite, different natural specimens may show 

 considerable differences. 



Table III gives the results in tabular form. The only mar- 

 casites not of questionable purity which we have ever had in our 

 hands, gave results which were almost identical. The results 

 on the Joplin marcasite also served to show the constancy of 

 the method ; the first two results were obtained about a year 

 later than the third with different standard solutions. The two 

 determinations on Elba pyrite No. 1 were also made a year 

 apart. The determinations for synthetic pyrite were made on 

 two different preparations. 



The variations on different occurrences of the same mineral 

 species are due in part, if not entirely, to impurities. Thus the 

 Leadville pyrite contains copper probably as chalcopyrite 

 which Stokes found to give low results. Elba pyrite No. 1 is 

 the purest we have ever found ; the quantity at our disposal 

 was unfortunately insufficient for this investigation. Elba 

 pyrite No. 2 contained a little cobalt, but hardly sufficient to 

 cause the irregularity noticed. It was selected from a large 

 sample which contained considerable hematite, traces of which 

 were probably retained by the sulphide even after purification. 

 That the suspicion was well founded was proved by the fact 

 that after the ground mineral had been treated for two da} T s at 

 200° with two per cent sulphuric acid saturated with hydrogen 

 * J. Am. Chem. Soc, xxxiii, 1104, 1911. 



