GEOPHYSICAL LABORATORY, 105 



both. Micro-organisms may have been active in the formation of pyrite and 

 marcasite by giving rise to hydrogen sulphide. 



Pyrrhotite was formed by the decomposition of pyrite or by heating 

 marcasite, or by heating iron with excess of sulphur. The dissociation of 

 pyrite into pyrrhotite and sulphur is readily reversible. At 565° (about) 

 pyrite and pyrrhotite are in equilibrium with the partial pressure of sulphur 

 in HgS, which here amounts to about 5 mm. (data of Preunner and Schupp) ; 

 at 550° in hydrogen sulphide, the pyrrhotite passes into pyrite, and at 575° 

 the reverse action proceeds. At about 665° the evolution of sulphur from 

 pyrite becomes rapid and a marked absorption of heat results. The pressure 

 of the sulphur-vapor here reaches one atmosphere. 



Pyrrhotite is of variable composition. Its composition at any temperature 

 depends on the pressure of sulphur-vapor in which it is heated. Though it 

 has not been found feasible to vary the temperature and pressure inde- 

 pendently, a series of products were prepared by first decomposing pyrite 

 and then reheating the resulting material to various measured temperatures 

 in hydrogen sulphide and then chilling in the same or cooling in nitrogen. 

 The products lowest in sulphur were obtained in the latter way. These 

 products all resemble natural pyrrhotite in physical and chemical properties. 

 Their specific volumes vary continuously with composition and pyrrhotite is 

 therefore to be regarded as a solid solution of sulphur in ferrous sulphide. 

 The maximum percentage of dissolved sulphur in synthetic pyrrhotite was 

 6.04 per cent at 600°. By extrapolation the saturated solution at 565°, 

 below which point pyrite forms, was estimated to be 6.5 per cent. This 

 corresponds closely to the maximum percentage of sulphur reported in 

 natural pyrrhotite. 



Equilibria between pyrrhotite and the partial pressure of sulphur in dis- 

 sociated hydrogen sulphide were determined at ditTerent temperatures, by 

 sufficiently long heating and then rapid cooling. The dissolved sulphur 

 varied under these conditions from 6.0 per cent at 600° to 2.0 per cent at 

 1300°. The curve shows a discontinuity at the melting temperature, at the 

 beginning of which there is a sudden decrease in the percentage of sulphur. 



The melting-point of pure ferrous sulphide could not be exactly deter- 

 mined, because the compound dissociates at high temperatures into its ele- 

 ments. By heating it in a vacuum this dissociation was placed beyond doubt, 

 though the dissociation was so slow that the melting-point could be located 

 approximately. It may safely be put at 1170° d= 5°. In hydrogen sulphide, 

 the melting temperature is raised because the solid solution thus formed con- 

 tains more sulphur than the first portion of liquid to which it melts. The 

 limits of the melting interval can not be determined as yet, but the maximum 

 heat absorption falls at 1183°. In one atmosphere of sulphur-vapor this 

 temperature rises to 1187°. 



Crystals of pyrrhotite, the measurements of which are recorded under 

 the crystallographic study, were repeatedly formed at various temperatures 

 between 80° and 225° by the action of hydrogen sulphide on slightly acid 

 solutions of ferrous salt containing some ferric salt. The product usually 

 contained some crystals of disulphide. 



Troilite is only the end-member of the pyrrhotite series and not a distinct 

 mineral species. Thus far it has not been prepared free from metallic iron. 



(25) Studies in ore deposition, with special reference to the sulphides of iron. E. T. 

 Allen. Mining and Scientific Press, 103, 413. 191 1. 



A briefer presentation of No. 24 for the use of engineers. 



