170 Allen, Crenshaw, Johnston, and Lar sen — 



Wdhler, Kammelsberg and others, but for the most part by 

 methods which throw little light on their formation in nature.* 

 Method of study.— -In the investigation of this subject the 

 synthetic method has been largely followed, while the most 

 significant properties, reactions and relations of the substances 

 have been studied. Some of the material will be chiefly of 

 chemical interest, but the effort has been made to give special 

 attention to the chemical geology involved and to this end the 

 authors have consulted frequently with several eminent geolo- 

 gists who are specialists in this field. The experience and 

 suggestions of these scientists have been of great value and will 

 be duly acknowledged in the proper place. No optical studies 

 were of course possible on opaque minerals, but crystallo- 

 graphic measurements were made and such microscopic studies 

 as the character of the material admitted of. 



I. The disulphides (FeSJ Pyrite and Marcasite. 



Two disulphides of iron are known — pyrite, hard, lustrous, 

 brassy -yellow, of sp. gr. 5*027t at 25°, and crystallizing in 

 the regular system; and marcasite, yellowish grey in color, 

 a little softer than pyrite, of sp. gr. 4*887f at 25° and crystal- 

 lizing in the orthorhombic system. Both minerals are almost 

 insoluble in hot hydrochloric and dilute sulphuric acids, and 

 both are decomposed by nitric acid. Both are slowly oxidized 

 by free oxygen, the products being, according to conditions, 

 sulphur dioxide and ferrous sulphate, or sulphuric acid and 

 ferrous or ferric sulphate, — sometimes, in fact, sulphuric 

 acid and ferric hydroxide. The conditions have not been 

 investigated completely, though certain definite statements 

 can be made at the present time. In a closed vessel containing 

 air, i. e., with an excess of sulphide, the products are sulphur 

 dioxide and ferrous sulphate. When the minerals are heated 

 to 100° in air or ground dry in a mortar, these are the first 

 products at least. The- sulphur dioxide changes pretty readily, 

 of course, to sulphuric acid. When kept in contact with 

 air and moisture at 75° most of the iron takes the form of 

 ferric hydroxide. Many oxidizing agents, important among 

 which, from a geological standpoint, are ferric sulphate and 

 copper sulphate solutions, change pyrite and marcasite into 

 ferrous sulphate, sulphur and sulphuric acid. In nitric acid 

 of 1*4 sp. gr., powdered pyrite dissolves completely, while 

 marcasite separates sulphur. f Both minerals are so nearly 



* An exception should be made of the excellent work of Senarinont, Ann. 

 Ch. Phys., xxxii, 129, 1851. 



f These specific gravities were determined on very pure natural minerals, 

 the analyses of which are given on p. 177. 



% Brush and Penfield's Determinative Mineralogy, 15th ed., p. 252. 



