Mineral Sulphides of Iron. 205 



color but continued to decrease in weight the longer it was 

 heated. These facts are graphically shown in fig. 11. Pyrite 

 under the same conditions gave inside of a few hours a per- 

 ceptible quantity of pyrrhotite, which was proved by testing 

 with warm hydrochloric acid. At 565° the pyrite formed in 

 several hours only a doubtful trace of pyrrhotite, if any. 

 Between 550° and 575°, therefore, the two curves 1, 1 and 

 2, 2 cross, and, at that point, about 565°, at a pressure of 

 about 5 mm of sulphur, pyrite should be in equilibrium with a 

 pyrrhotite containing about 6*5 per cent of dissolved sul- 

 phur. How this quantity would vary with conditions we 

 do not yet know, though, as we have just seen, the solu- 

 tion of sulphur in ferrous sulphide of maximum concen- 

 tration found in nature, formed presumably from water solu- 

 tions, does not vary much from it. 



The change from pyrite to pyrrhotite is, then, a reversible 

 reaction, FeS a < > FeS(S) x + (l — %)$. Since the system 

 contains a gaseous phase, the temperature at which the change 

 occurs is manifestly dependent on pressure. 



Dissociation point. — It has been previously stated that 

 pyrite undergoes dissociation into pyrrhotite and sulphur and 

 that this dissociation is detectable at 575° after the lapse of 

 several hours, when the heating is done in hydrogen sulphide. 

 If the heating is continued at a moderate rate (2° per min.) 

 a strong absorption of heat manifests itself at about 665°. 

 Here, under these conditions of heating, the dissociation there- 

 fore becomes suddenly accelerated, and it is probable that the 

 pressure of the escaping sulphur reaches one atmosphere. 

 The solid phases pyrite and pyrrhotite, i. e., the saturated solu- 

 tion of sulphur in ferrous sulphide, should at a fixed tempera- 

 ture maintain a fixed pressure. As a matter of fact, the point 

 is not sharp ; the temperature gradually rises through an inter- 

 val of about 20°. This is probably due to the formation of a 

 coating of pyrrhotite on the pyrite grains, which retards the 

 dissociation, so that the system requires a gradually rising tem- 

 perature to maintain the pressure. The fact that undecom- 

 posed pyrite persists so tenaciously in the product seems to 

 support this view. A typical thermal curve for pyrite in this 

 region is shown in fig. 12. It is seen to have the same general 

 form as a melting point curve and the end of the heat absorp- 

 tion is more sharply marked than the beginning, though for 

 the reasons stated the latter has the greater significance. The 

 following data show that the absorption ends pretty uniformly 

 at about 685°: 



El. V 



6050 mv. 



= 686° 



a 



603V mv. 



= 684° 



El. S 



6083 mv. 



= 686° 



a 



6060 mv. 



= 684° 



