414 H. N. Stokes — Pyrite and Marcasite. 



Art. XL1T. — On Pyrite and Marcasite ; by H. K Stokes. * 



While pyrite and marcasite are usually readily distinguished 

 by their crystalline form, there remains a residuum, consisting 

 of massive or finely-grained concretionary material, in which 

 this is not possible. The light brass color of pyrite and the tin- 

 white of marcasite, which can be seen when the surfaces are 

 freshly cleaned with acid and compared with standard speci- 

 mens in a good white light, are not always readily made out in 

 concretions where the surface is rough and the mineral 

 frequently contaminated with other substances. The density 

 too may be misleading. According to Ramrnelsberg and 

 to my own determinations, pure marcasite has a density 

 of about 4-90,t while that of pyrite is 5'00 to 5*04. The 

 density of even well crystallized specimens of pyrite varies 

 very considerably, and as a criterion of the presence or 

 absence of marcasite is practically worthless, especially in mas- 

 sive or concretionary material. A series of crystallized pyrites, 

 which were shown by the method to be described below to be 

 free from marcasite, gave densities varying from 5*04 to 4*82, 

 while a pyrite concretion, also free from marcasite, gave only 

 4*56. A. A. JulienJ has employed the density to determine 

 the relative amount of pyrite and marcasite in mixtures, and on 

 this method he has based the hypothesis that most specimens 

 of these minerals, even when well crystallized, are intimate 

 mixtures of the two, passing into complete paramorphs. It 

 will be shown below that this view is untenable. 



The greater rapidity of oxidation is sometimes used to dis- 

 tinguish the minerals. While it is unquestionably true, that 

 under precisely similar conditions marcasite vitriolizes more 

 rapidly than pyrite, conclusions based on this fact have usually 

 failed to take into account the important factor of the ratio of 

 surface to mass ; a compact, brilliant, marcasite is stable while 

 a porous mass of pyrite vitriolizes readily. Only when we 

 know this factor can we draw any conclusion from the fact of 

 rapid vitriolization. 



Penfield has described a method for distinguishing the min- 

 erals, based on the fact that boiling nitric acid liberates sulphur 

 from marcasite but not from pyrite, a method which is clearly 

 not adapted to detecting pyrite in the presence of marcasite. 



* This paper is a condensation of Bulletin No. 186 of the United States Geolog- 

 ical Survey and is published by permission of the Director. 



f The density 4-80 given by Julien (Annals N. Y. Acad, Sci., vol. iv, 188*7, pp. 

 177, 210) is certainly too low. 



\ 1. c. pp. 166, 213. 



