192 Allen^ Crenshaw* Johnston^ and Larsen — 



product of surface conditions. The oxidation of either pyrite 

 or marcasite gives first a mixture of sulphuric acid and ferrous 

 sulphate which by further oxidation easily gives ferric sul- 

 phate. The action of hydrogen sulphide and atmospheric 

 oxygen simultaneously on the acid ferrous solution would 

 lead to the same goal. We have found how hydrogen sulphide 

 acting on acid solutions, especially in the cold, gives rise to 

 marcasite. We have also found that above 450° marcasite 

 could not form, thus further confirming geological deductions. 



Pyrite, being a stable form, probably crystallizes under a 

 considerably wider range of conditions than marcasite. The 

 evidence of synthetic study is that the formation of pyrite 

 is favored by high temperature and by solutions which con- 

 tain little or no free acid. In accord with these, we have the 

 following geological deductions. First, pyrite is a product of 

 hot springs. In the springs of Carlsbad, which have a tem- 

 perature of about 55° C, recent pyrite is observed.* The 

 waters contain sulphates and a trace of hydrogen sulphide, 

 and are slightly alkaline. The lagoons of Tuscany are deposit- 

 ing pyrite from their hot waters. Bunsenf found that the 

 hot vapors of the fumaroles of Iceland were gradually chang- 

 ing the ferrous silicate of the basalts into pyrite. 



More important geologically is the fact that the product of 

 deep veins by ascending waters is always pyrite, never marca- 

 site. Such waters are naturally hot, and commonly if not 

 always alkaline.^ We can now see that a separation of pyrite 

 from a magma is entirely possible, while the temperature of 

 any magma would doubtless be incompatible with the existence 

 of marcasite. 



The occurrence of pyrite and marcasite together. — Hintze§ 

 mentions thirty-one instances where pyrite and marcasite are 

 found intergrown or precipitated one upon the other. Stokes || 

 also tested a number of specimens which proved to be mix- 

 tures of pyrite and marcasite, some of them intergrown in con- 

 centric layers. In other places, e.g., in Joplin, Missouri, the 

 two minerals have been observed by us in the same hand spe- 

 cimen. T According to F. L. Ransome,** the two minerals 

 occur together, though perhaps not intergrown, in Goldfield, 

 Nevada. These facts show very strikingly not only the small 

 influence of nuclei in directing the form of the disulphide 



*Daubree, Geologie experimental, Paris, 1879, p. 93. 



fPogg. Ann., lxxxiii, 259, 1851. 



\ A hot acid solution in contact with carbonate or most silicate rocks 

 would first be neutralized and then become alkaline. 



§ Lehrbuch der Mineralogie, vol. i, pp. 724-778, 820-832. 



|| Loc. cit. 



1[ See W. S. T. Smith and Siebenthal, U. S. G. S. Folio 148. 

 • ** Private communication. Specimens were also submitted by Mr. 

 Raasome. 



