150 CARNEGIE INSTITUTION OF WASHINGTON. 



(29) The Stokes method for the determination of pyrite and marcasite. E. T. Allen and 

 J. L. Crenshaw. Am. Jour. Sci. (4), 38, 371-392 (1914). 



The Stokes method for determining pyrite and marcasite, alone or in mix- 

 tures, depends on the estimation of the iron dissolved when the finely ground 

 and purified sulphide is treated with a boiling standard solution of ferric 

 alum. The same pyrite or marcasite gives very constant values and the 

 influence of each in mixtures is additive, i. e., there exists a linear relation 

 between the iron dissolved and the composition of the mixture. The sum of 

 the errors usually amounts to about 1 per cent, reaching a maximum of 2 

 per cent. There are two important sources of error. First, there must be a 

 sufficient excess of the sulphide, which is many times greater (7 to 15) than 

 the amount required by theory. With such an excess the percentage of the 

 surfaces remains on the average nearly the same as the percentage by weight, 

 the basis on which the mixtures are made up. About 1 gram is sufficient for 

 250 c.c. of the standard solution. Secondly, the marcasite has a character- 

 istic tendency to flocculate and thus reduce its reacting surface. This diffi- 

 culty may be avoided by shaking the reacting mixture with pure quartz and 

 beads until the lumps of the powder are thoroughly disintegrated. Different 

 specimens of pyrite and marcasite give with the Stokes reaction values which 

 differ somewhat. The differences are due in some cases, if not in all, to the 

 presence of impurities. It is unfortunate that small quantities of impurities 

 which will reduce ferric iron or give up iron to the solution exercise a 

 serious influence. It is therefore not always possible to decide between a 

 natural pyrite and a pyrite containing several per cent of marcasite by the 

 Stokes reaction alone, nor to determine accurately the percentage of each in 

 a natural mixture. In an investigation on the conditions of formation of 

 pyrite and marcasite, this method has been very useful. 



The results with the Stokes method plainly indicate that each mineral 

 behaves in a mixture of the two just as it does alone; each appears to reduce a 

 quantity of solution which is proportional to its surface; and each appears to 

 reduce the solution at practically the same rate. The rates at which the 

 sulphides are decomposed is quite different for the two minerals, because more 

 of marcasite than of pyrite is required to reduce a given quantity of ferric 

 iron. The ratio of these rates is not far from 1 : 2.5. 



That ferric sulphate dissolves from pyrite a smaller quantity of iron than 

 it does from marcasite means simply that more reduction is effected by sulphur 

 in the case of pyrite; in other words, that more of the sulphur in pyrite is 

 oxidized. Stokes considered only the relation of p, the percentage of sulphur 

 oxidized, to y, the percentage of pyrite in the sulphide mixture. We have 

 shown that this curve is a hyperbola. This characteristic behavior of pyrite 

 and marcasite towards oxidizing agents is probably general. It has been found 

 by other observers that nitric acid and hydrogen peroxide both oxidize more 

 of the sulphur in pyrite under the same conditions. 



(30) Stokes Methode zur Bestunmung von Pyrit und Marcasit. E. T. Allen und J. L. Cren- 



shaw. Z. Anorg. Chem., 90, 81-106 (1914). 



A German translation of "The Stokes method for the determination of 

 pyrite and marcasite" (Am. Jour. Sci. (4), 38, 371-392: 1914). Reviewed 

 under No. 29 above. 



(31) Effect of temperature and acidity in the formation of marcasite (FeSj) and wurtzite 



(ZnS) ; a contribution to the genesis of unstable forms. E. T. Allen and J. L. 



Cren.shaw. Microscopic study by H. E. Merwin. Am. Jour. Sci. (4), 38, 



393-431 (1914). 



Our former results on the genesis of marcasite and wurtzite have been 



reinvestigated, the former conclusions have been confirmed, and new data 



