216 Allen, Crenshaw, Johnston, and Larsen — 



by the fact that marcasite cannot exist above 450°. The forma- 

 tion of pyrite in deep veins and hot springs is explained by the 

 fact that the waters from which it came were alkaline. The 

 marcasite of surface veins was probably formed from cold 

 acid solutions, while mixtures of marcasite with pyrite were 

 probably conditioned by higher temperature (up to 300°), 

 or the presence of less acid, or both. Micro-organisms may 

 have been active in the formation of pyrite and marcasite by 

 giving rise to hydrogen sulphide. 



5. Pyrrhotite was formed by the decomposition of pyrite or 

 heated 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 equi- 

 librium with the partial pressure of sulphur in II 2 S, 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 evolu- 

 tion of sulphur from pyrite becomes rapid and a marked 

 absorption of heat results. The pressure of the sulphur vapor 

 here probably reaches one atmosphere. 



6. 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 independently, a series of 

 products were prepared by first decomposing pyrite and then 

 reheating the resulting material to various measured tempera- 

 tures in hydrogen sulphide and finally chilling in the same or 

 cooling in nitrogen. The products lowest in sulphur were 

 obtained in the latter way. These products all resemble natu- 

 ral 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 pj^rrhotite 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 sul- 

 phur reported in natural pyrrhotite. 



7. Equilibria between pyrrhotite and the partial pressure of 

 sulphur in dissociated hydrogen sulphide were determined at 

 different temperatures, by sufficiently long heating and then 

 rapid cooling. The dissolved sulphur varied under these con- 

 ditions 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 percent- 

 age of sulphur. 



