Mineral Sulphides of Iron. 217 



8. The melting point of pure ferrous sulphide could not be 

 exactly determined because the compound dissociates at high 

 temperatures into its elements. By heating it in a vacuum 

 this dissociation was placed beyond doubt, though the dissocia- 

 tion was so slow that the melting point could be located 

 approximately. It may safely be put at 1170°rb5°. In hydro- 

 gen 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 cannot be determined as yet, but the maximum heat 

 absorption falls at 1183°. In one atmosphere of sulphur vapor 

 this temperature rises to 1187°. 



9. Crystals of pyrrhotite, the measurements of which are 

 recorded under the Crystallographic Study, were repeatedly 

 formed at various temperatures between 80° and 225° by the 

 action of hydrogen sulphide on slightly acid solutions of ferrous 

 salt containing some ferric salt. The product usually con- 

 tained some crystals of disulphide. 



10. The crystallographic study confirms the work of Hinne 

 and Boeke and others, that there are two crystal forms of 

 pyrrhotite. The high temperature form, a-pyrrhotite, appears 

 to be orthorhombic and the axial ratios vary from a\o : c = 

 0-5793 : 1 : 0*9267 to 0-5793 : 1 : 0-9927, depending on the amount 

 of the dissolved sulphur present (?). The habits of the crys- 

 tals are shown in figs. 17, 18 and 20, Ft. III. ^-pyrrhotite is 

 hexagonal, and g varies from 0*8632 to 0-8742. The crystal 

 habit is shown in figs. 22 and 23, Pt. III. 



The crystal constants of natural pyrrhotite can not be 

 assumed to be invariable, since the composition of the mineral 

 is not constant, and the crystal angles of the synthetic mineral 

 are variable. The lack of agreement among mineralogists 

 regarding the crystal system to which pyrrhotite belongs can 

 be explained on the theory that there are two forms of pyrrho- 

 tite. 



11. Troilite is only the end member of the pyrrhotite series 

 and not a distinct mineral species. Thus far, it has not been 

 prepared free from metallic iron. 



In conclusion, the authors wish to express their hearty 

 thanks to Dr. Carl Alsberg and Dr. Oswald Schreiner for 

 important references, and to Dr. F. L. Ransome, Dr. W. H. 

 Emmons and Mr. C. A. Davis for valuable geological informa- 

 tion, and especially to Mr. W. Lindgren, not only for placing 

 at our disposal the generalized results of his extensive experi- 

 ence, but for reading this paper and making a number of 

 important suggestions. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, Dec. 1, 1911. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 195.— March, 1912. 

 15 



