86 BtPLiLETIN 184, UNITED STATE'S NATIOiN!AI> MUSEXJM 



Ferrous sulphide. — Sulphur in meteoric ii'ons occurs chiefly as 

 troiUte. Though not an appreciable factor in the general structure, 

 it nevertheless presents interesting features. 



The iron-sulphide (Fe-FeS) constitution diagram worked out by 

 Treitschke and Tammann (1906), of which an illuminating discussion 

 is given by Rinne and Boeke (1907), shows transformations similar 

 to those of iron, with gamma, beta, and alpha phases. The pure 

 sulphide melts at 1,300°. The melting point of the iron and iron- 

 sulphide solution (or its point of solidification) varies according 

 to its proportions. The eutectic temperature for a solution of 84 

 percent iron and 16 percent sulphide is 970°, which also marks the 

 gamma-beta transformation. The beta-alpha transformation takes 

 place at the very low point of 130°. 



Age of troilite inclusions.- — The conclusion of Rinne and Boeke 

 was that the h'on-bearing sulphide separated from the melt in minute 

 droplets, consolidating into the larger droplike or nodular forms in 

 which troilite most commonly occurs, from which as cooling pro- 

 ceeded the dissolved iron was progressively rejected. Even apart 

 from the diagram, it is clear that such inclusions are an original 

 segregation from the melt, because they are independent of the 

 Widmanstatten structure and therefore obviously older. As cooluig 

 proceeds the troilite masses continue to reject more and more nickel- 

 iron in the solid state, which produces the band of "swathing kama- 

 cite" that usually surrounds such inclusions when they are of any 

 considerable size, against which the octahedral structure abuts 

 unconformably. 



The eutectic temperature of iron and troilite (970°) is 300° to 

 500° above the beginning of the gamma-alpha transformation in an 

 alloy in the composition range of octahedrites. Thus the troilite 

 necessarily would have solidified before the production of the octa- 

 hedral structure began. At 970° the nodules would be iron 16 per- 

 cent, sulphide 84 percent. But inasmuch as the saturation point 

 of sulphide in alpha iron is stated to be only 2 percent, most of the 

 iron in the eutectic would be rejected as it cooled from 970° to room 

 temperature; in effect the nodule as it shrinks leaves behind it a 

 thickening skin of kamacite. 



In its final form the analyses indicate that troilite contains 3 to 

 4 percent of iron in solid solution. This proportion, somewhat 

 higher than the above-mentioned saturation ratio of 2 percent, might 

 be due to the presence of iron in the form of occluded particles re- 

 jected along grain boundaries in the troilite and not actually in solid 

 solution. 



In their discussion of the ternary iron-nickel-sulphide system, 

 Vogel and Tonn (1930) also reach the conclusion that troilite, both 



