GElOPHYSICAIv LABORATORY, IO5 



under proper conditions. Measurable crystals of pyrrhotite have been formed 

 by the action of HgS on ferrous salts at temperatures between 75° and 200°. 

 As proved by others, pyrrhotite undergoes a transformation at about 138°. 

 Products of varying composition were obtained by heating fused material, 

 coarsely powdered, to different temperatures in HjS and then cooling in 

 nitrogen ; they ranged in composition from 36.86 per cent to 39.49 per cent 

 of sulphur. In HgS all products melt at about 1180° where the composition 

 approaches FeS, though rather more than 2 per cent of sulphur is tenaciously 

 retained. In color and behavior toward hydrochloric acid, the products agreed 

 with natural pyrrhotite. The specific gravity varied from 4.769 to 4.598. The 

 specific volume plotted against composition (percentage of S over the ratio 

 Fe : S = I : I ) is practically a straight line. This indicates that pyrrhotite 

 is a solid solution of sulphur in FeS, and troilite is simply the FeS end of 

 the series, without excess of sulphur because it was formed in a matrix of 

 metallic iron. Experiments on the crystal form of pure FeS and on the 

 influence of the vapor pressure of sulphur (dissociated H2S) on the compo- 

 sition of pyrrhotite are still in progress. 



2. Marcasite and pyrite: Marcasite has been synthesized at temperatures 

 of about 200° by the action of HjS on ferric salts. Measurable crystals 

 showing the proper axial ratio and striations demanded by orthorhombic 

 symmetry were obtained. Some pyrite always seems to form at the same 

 time. Pure pyrite has been obtained by the addition of sulphur to pyrrhotite 

 and also by the action of HjS on ferric hydroxide above 100°. 



Marcasite is an unstable form passing into pyrite with evolution of heat. 

 The lowest temperature at which the change can be detected is 450°. Even 

 in the presence of sulphuric acid none takes place at 350°. The inference is 

 that all marcasite has formed below 450°. It has not proved possible to 

 change marcasite by pressure, cold and dry, at 10,000 atmospheres. It is not 

 improbable, however, that this pressure would change marcasite into pyrite 

 at temperatures lower than 450°. Stokes's method was used for the identifi- 

 cation of marcasite and pyrite. 



Pyrite seems to be stable (away from the air) up to about 500°, where, 

 in an atmosphere of HgS, it begins to lose sulphur. In the presence of free 

 sulphur (or dissociated HoS) it is evident that the point of dissociation 

 would depend upon the pressure and might, in deep-seated magmas, be much 

 above 500°. 



