Decomposition of Iron Pyrites. 203 



pyrrliotite. Under a microscope, exceedingly fine-grained 

 and splendent, with a few imperfect cubes of pyrite distin- 

 guishable, having a platy structure and octahedral modifications 

 of their solid angles. Decomposition : no weathered surface 

 upon the specimen, but the fracture reveals, under the micro- 

 scope, a general blue and red iridescence. 



No. 116. Marcasitic pyrite. Mexico. An irregular frag- 

 ment of an '' Aztec mirror," loaned to me for examination by 

 Mr. George F. Kunz ; dimensions about 83 by 45 mm., and 15 

 mm. in thickuess, and weight 62.7 grams. The face has been 

 ground smooth and still retains considerable polish ; the reverse 

 is roughly convex, apparently by the rubbing off and rounding 

 of the edges of crystalline faces of the pyritohedron. Color of 

 the polished face of the mirror, pale yellowish white ; of the re- 

 verse, rather dull greenish yellow ; of the fresh fracture, gray- 

 ish white, inclining to yellowish, but not very bright. 



Under a loup or low power of the microscope, three materials 

 may be distinguished. 



1. Marcasitic pyrite, largely predominating in minute cubes, 

 and also in thin plates (possibly of marcasite), mostly a little 

 less than 0.1 mm. in diameter, and bright tin-white on fracture. 



2. Dark brown to black particles, lying in great abundance 

 in the interstices of the preceding crystals, and which seem to 

 consist of iron-oxide, and perhaps carbonaceous matter or other 

 metallic sulphides ; this substance has refused to take the polish, 

 and produces a minute dark pitting over the face of the mirror. 



3. Over the surface, several minute branching veinlets run, 

 about 0.1 mm. thick, pale brass-yellow and brilliant, evidently 

 of pure pyrite. 



The specimen strikes fire with steel and yields a dull reddish 

 black streak or powder. It possesses a special interest, in re- 

 lation to the Peruvian mirrors of marcasite, the historic pierre 

 des Incas, to which reference has already been made. 



