172 Decomjjosition of Iron Pyrites. 



by the rectangular, nearly square, tcrmiiuil faces (OP) of simi- 

 larly arranged rhombic plates and triangular twins. 1'he massive 

 mineral and its dark greenish brown powder both emit a strong- 

 sulphurous odor, which however does not blacken paper moist- 

 ened with lead-acetate. Streak, brownish black, very slightly 

 greenish. Decomposition : the fresh fracture soon assumes a 

 yellowish tinge : a yellowish brown film of iron-oxide covers the 

 hollows in the botryoidal surface. 



No. 14. Marcasite. Dover Cliffs, England. A brilliant 

 cluster of spear-headed crystals — broad, striated twinned plates, 

 — resembling those of Xo. 11, imbedded in light gray chalk. 

 Grayish white and bright on fracture. Decomposition : a slight- 

 ly iridescent tarnish, with red, blue and yellow colors. 



No. 15. Marcasite. Canada. A compact crnst, steely white 

 and brilliant on fracture, with curved finely fibrous structui'e ; 

 mixed with sphalerite. Under the microscope, the fracture 

 shows the tin-white splendent faces of striated columns. Decom- 

 position : bronze-colored tarnish, yellow or iridescent in places, 

 with little pockets of reddish iron-ochre. 



No. 16. Marcasite. Galena, Illinois. A thick crust, grayish 

 white and brilliant on fracture, with radial fibrous structure. 

 Surface covered by grouping of twinned rhombic ])risons, wedge- 

 shai)ed and composite, made up of sti-iated six-sided plates. 

 Decomposition : a beautiful orange and blue iiidescence, with 

 high lustre. 



No. 17. Marcasite. Galena, Illinois. A large stalactite, 

 whose core consists of sphalerite and i)yrite, encrusted by a thick 

 coating of marcasite, in large radiating tooth-like rhombic pyra- 

 mids, highly modified ; grayish white and brilliant on fracture. 

 The coffin-shaped faces of the brachydome (m p co) are invaria- 

 bly even and bi'illiant, but little marked by lines of composite 

 structure or of cleavage. These faces and those of the basal 

 pinacoid (OP) are unusnally hard, about 6.5, being just scrat- 

 ched by a file and by quartz, but not by orthoclase. The faces 

 of the macrodome are less bright and commonly marked by 

 rhombic composition-lines ; these are evidently softer, about 6 

 in the scale of hardness, being deeply scored by a file and even 

 just scratched by orthoclase. Decomposition : dull bronze-color- 

 ed tarnish, without styptic taste. 



