H. N. Stokes — Pyrite and Marcasite. 415 



There is no method hitherto known by which even a qualita- 

 tive determination of the two is possible in mixtures. 



In connection with the study of the action of ferric salts on 

 sulphides, J was struck by the quantitative difference of the 

 behavior of pyrite and marcasite, and out of this has been 

 developed the following method for distinguishing them and 

 for determining them quantitatively in mixtures. 



That ferric salts oxidize the sulphur of pyrite to sulphuric 

 acid was observed by de Koninck* and the same has since been 

 noted by other observers, without an attempt having been 

 made to follow the reaction quantitatively. The study which 

 I have made of this reaction shows that under uniform and 

 easily controllable conditions. the percentage of the total sul- 

 phur oxidized is very constant for each mineral, but differs 

 greatly in the two, and in mixtures is an index of the amount 

 of each constituent present. To avoid tedious gravimetric 

 determinations I employ a standard solution of ferric ammo- 

 nium alum containing 1 gram Fe 7// per liter, with 4 grams free 

 sulphuric acid to prevent the formation of basic salts. An 

 excess of the carefully prepared mineral is boiled with this 

 solution under absolute exclusion of air until the reduction of 

 the ferric salt is practically complete. The reaction may be 

 regarded as taking place in two stages : 



(1) FeS Q + Fe 2 (S0 4 ) 3 =3FeS0 4 + 2S. 



(2) 2S + 6Fe 9 (S0 4 ), + 8H a O= 12FeS0 4 + 8H 2 S0 4 . 



Since the active mass of the solid phase is constant, it matters 

 not how much mineral be taken for a given volume of the 

 solution provided it be in excess, nor is it necessary to know 

 the absolute quantities of iron and sulphur involved in these 

 reactions, or the strength of the permanganate solution. It 

 suffices, in deducing an expression for the oxidizsd sulphur, to 

 employ symbols expressing the permanganate equivalent of the 

 iron, and the symbols used represent simply the volumes of 

 permanganate consumed by a given volume of the solution. 

 For a given volume let 



a = iron in the original solution, 



b ±= resulting ferrous iron, 



c = resulting total iron. 

 Then 



c — a = increment of iron resulting from decomposition of FeS 2 , 

 and 



2^ (« 



\55-60/ V 



■ a) = total sulphur in decomposed sulphide. (A) 



* Annals Soc. Geol. Belgique, vol. x, 1883. p. 101; Zeitschr. anorg. Chem., vol. 

 xxvi, 1901, p. 123. 



