abstracts: mineralogy 93 



MINERALOGY. — The Stokes method for the determination of pyrite and 

 marcasite. E. T. Allen and J. L. Ceenshaw. Am. Jour. Sci. 

 (4), 38: 371-392. 1914. 



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

 mixtures, depends on the estimation of the iron dissolved when the 

 finely ground and purified sulphide is treated with a boihng standard 

 solution of ferric alum, The same pyrite or marcasite gives very con- 

 stant 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 sul- 

 phide, 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 cc. of the standard solution. Secondly, the marcasite has a char- 

 acteristic tendency to flocculate and thus reduce its reacting surface. 

 This difficulty 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 un- 

 fortunate that small quantities of impurities which will reduce ferric 

 ron 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 are quite different for the 

 two minerals, because more of marcasite than of pyrite is required to re- 

 duce 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 



