abstracts: mineralogtcal chemistry 11 



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 anytempera- 

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

 Tho it has not been found feasible to vary the temperature and pres- 

 sure independently, a series of products were prepared by first decom- 

 posing pyrite and then reheating the resulting material to various meas- 

 ured 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 continu- 

 ously with composition and pyrrhotite is therefore to be regarded as 

 a solid solution of sulphur in ferrous sulphide. The maximum per- 

 centage 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 pyrrho- 

 tite. 



Equilibria between pyrrhotite and the partial pressure of sulphur in 

 dissociated hydrogen sulphide were determined at different tempera- 

 tures, 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 

 elements. By heating it in a vacuum this dissociation was placed be- 

 yond doubt, though the dissociation was so slow that the melting-point 

 could be located approximately. It may safely be put at 1170° = fc 5°. 

 In hydrogen sulphide, the melting temperature is raised because the 

 solid solution thus formed contains 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 tempera- 

 tures between 80° and 225° by the action of hydrogen sulphide on slightl 





