372 Allen and Crenshaw — Stokes Method for the 



In a recent investigation by the authors, on the genetic con- 

 ditions of certain allotropic forms, such a method became 

 essential. We had already made some use of the Stokes 

 method for identifying synthetic products, but had never care- 

 fully tested its accuracy. This has now been done ; two serious 

 sources of error have been detected and obviated, and some 

 facts and relations of much more general interest have been 

 discovered and worked out, in connection with it. 



II. The Stokes Method.* 



The Stokes method depends on the difference in the behavior 

 of the two minerals when boiled with a standard solution of 

 ferric sulphate. The reaction takes place in two stages, thus : 



(a) FeS„ + Fe,(S0 4 ) 3 = 3FeS0 4 + 2S. 



(b) S +"3Fe 2 (S0 4 ) 3 + 4H 2 = 6FeSG 4 + 4H 2 S0 4 . 



Both minerals give the same products, viz., ferrous salt, 

 sulphur and sulphuric acid ; their difference is manifested in 

 the relative quantities of the products, for experiment shows 

 that about 52 per centf of the sulphur in pyrite is oxidized to 

 sulphuric acid, while only about 12 per cent of the sulphur in 

 marcasite is oxidized. This behavior is not entirely unique. 

 Penfield^: found that concentrated nitric acid dissolved pyrite 

 completely (i. e., all the sulphur is oxidized), while it leaves 

 with marcasite a copious residue of sulphur. More recently, 

 Arbeiter§ has made a long series of careful experiments with 

 hydrogen peroxide and hydrochloric acid, in which he finds 

 that marcasite always gives more free sulphur than pyrite 

 under the same conditions. As the reagents are diluted the 

 relation between the sulphur oxidized and the sulphur liberated 

 approaches a simple constant ratio characteristic for each 

 mineral. Arbeiter very plausibly infers that these phenomena 

 are dependent on a difference in constitution between pyrite 

 and marcasite. 



To identify either pyrite or marcasite, Stokes determined 

 the percentage of sulphur which was oxidized by a standard 

 ferric sulphate solution. Before proceeding to show how the 

 composition of a mixture of the two is determined, it will be 

 best to explain the chemistry of the process in greater detail. 

 The standard ferric alum solution, which contains close to 1 g. 



*H. N. Stokes, Bull. U. S. Geol. Survey, 186. 



f The numbers vary somewhat in different natural specimens. The 

 numerical data here and in the sequel belong to a pyrite from Roxbury, 

 Connecticut. Stokes' values were 60 - 5 and 16-5-18 *0 for pyrite and mar- 

 casite respectively. These variations will be discussed farther on. 



% Brash and Penfield, Determinative Mineralogy, 15th ed., p. 252. 



§ Min. Chem. Untersuch. an Markasit, Pyrit und Magnetkies. Inaugural 

 Dissertation, Breslau, 1913. 



