Decomposition of Iron Pyrites. 



309 



Iron disulphide, Fe S a , containing in 100 parts : iron, 46.07, 

 and sulphur, 53.33. 



This combination is dimorphous, i. e., assumes two forms of 

 crystallization in nature, and thus constitutes two minerals, 

 marcasite and pyrite, of very common occurrence. There is 

 also another natural form of iron sulphide, the mineral pyrrho- 

 iite or magnetic pyrites, whose composition and formula inter- 

 vene between those of the pro to- and disulphides. The dis- 

 tinctive characteristics of these three kinds of pyrites may be 

 tabulated as follows : 





Pyrrlwtite. 



. Marcasite. 



Pyrite. 



Composition, 



Fe 5 S 6 to Fe 16 S 17 , 

 and commonly 

 nickeliferous. 



FeS 2 . 



Fe S' 2 . 



Hardness, 



3.5—4.5. 



6.-6.5. 



6.-7. 



With steel, 





Striking fire imper- 



Striking fire readi- 







fectly, with strong 



ly, with weak sul- 







sulphurous odor. 



phurous odor. 



Specific gravity, 



4.4—4.68. 



4.68—4.85. 



4.74—5.19. 



Color, 



Bronze-yellow to 



Grayish white to 



Golden to pale 





steel tomback- 



bronze-yellow. 



brass-yellow. 





brown. 







Streak, - 



Grayish black. 



Greenish gray to 

 brownish black. 



Brownish black. 



Fracture, - 



Uneven. 



Uneven. 



Conchoidal to un- 

 even. 



Grains, - 



Compact and 



Columnar struc- 



Often fibrous and 





magnetic. 



ture. 



radial. 



Crystals, - 



Bare : hexagonal, 



Common : ortho- 



Abundant ; iso- 





generally tabu- 



rhombic, often in 



metric, cubes, py- 





lar, and mag- 



striated twins, 



ritohedra, octahe- 





netic. 



toothlike or crest 

 ed forms, etc. 



dra, etc. 



Ignition in 



Unchanged. 



Yielding sublimate 



(Like marcasite.) 



closed tube, 





of sulphur and 

 magnetic residue. 





In nitric acid, - 



Insoluble. 



Soluble with sepa- 

 ration of sulphur. 



(Like marcasite.) 



In hydrochloric 



Soluble with sepa- 



Insoluble. 



Insoluble. 



acid, - 



ration of sulphur 

 and hydrogen sul- 

 phide. 







Alteration, 



Iridescent tarnish. 



Iridescent, often 



Iridescent, efflo- 







efflorescenl. 



rescent, or hepatic. 



