H. 'JV. Stokes — Pyrite and Marcasite. 419 



vapor for half a minute and then to hydrogen sulphide gas ; the 

 chalcopyrite is blackened, while the iron sulphide remains 

 bright. I have used the same method to detect and establish 

 the nature of minute grains of chalcopyrite inclosed in rocks. 



My examination of various samples of doubtful nature, 

 especially of concretions, shows that the finely fibrous speci- 

 mens passing as marcasite are very commonly pyrite. 



Dr. Julien has very kindly given me a number of the iden- 

 tical specimens of supposed marcasitic pyrite and paramorphs 

 of marcasite after pyrite which were described in his paper.* 

 I have found that those which show regular crystallization are 

 actually pyrite free from marcasite, notwithstanding their 

 density would lead to the conclusion that they were either pure 

 marcasite or a mixture. So far as my results go, there is no 

 evidence for the existence of such mixtures or that the sup- 

 posed paramorphs sometimes described are anything more than 

 replacement or incrustation pseudomorphs. I have, however, 

 found a few specimens of marcasite which enclose pyrite which 

 cannot be detected by a lens, and which was probably simulta- 

 neously deposited. 



A . P. Browrts hypothesis. — A. P. Brownf has published 

 experiments which consisted in heating pyrite and marcasite in 

 sealed tubes at 200° with cupric sulphate solution, according to 

 which marcasite gives up its iron wholly in the ferrous form, 

 while pyrite gives a mixture containing one-fifth ferrous and 

 four-fifths ferric iron. From this he concluded that the iron 

 in marcasite is wholly ferrous, while in pyrite it is four-fifths 

 ferric. I have conducted a series of similar experiments with 

 pyrite and marcasite and cupric sulphate, under the same con- 

 ditions, in which the greatest care was taken to eliminate 

 oxidation and to determine both ferrous and ferric iron in the 

 solution and the precipitate. My results all agree in indicating 

 that the pyrite is more slowly attacked than marcasite, but that 

 the iron is found as ferrous iron only in the solution, and in the 

 precipitate as ferric iron mixed with cuprous oxide and cuprous 

 sulphide. The results are 



-p, . j 62*5— 69*9 per cent ferrous iron 



*?y ( 34*8 — 30*1 per cent ferric iron. 



■j-, ., ( 58 # 2 — 66*6 per cent ferrous iron 



Jbor marcasite < ,, „ OG . r . - . . 



( 41-8— 33*4 per cent feme iron. 



It appears therefore, that there is no essential difference 

 between the decomposition products of pyrite and marcasite, 

 while Brown's view requires 20 per cent ferrous iron for 



* Annals N. Y. Acad. Sci., vol. iv, 1887, pp. 116, 204. 



f Proc. Am. Philos. Soc, vol. xxxiil, 1894, p. 225 ; Chem. News, vol. lxxi, 1895, 

 p. 179. 



