184 Decomposition of Iron Pyrites. 



A granular material of rather cellular structure, consisting of 

 small grains, finely fibrous and radial on cross-fracture, yellow- 

 ish white, and grayish white, with cavities drusy with micro- 

 scopic crystals of marcasite. Grains of quartz are occasionally 

 seen enclosed. Decomposition: a bronze-colored tarnish, and, 

 in a damp atmosphere, a rapid and abundant efflorescence of 

 white yitriol, ferrous sulphate, which rapidly corrodes the label 

 of the specimen. 



No. 21. (Crystallized surface). Pyrite with marcasite. A 

 loose aggregate of cubes of yellowish white j^yrite, of composite 

 scaley structure, with drusy cavities in their interstices, lined 

 by the characteristic thougli minute, deei)ly furrowed, rhombic 

 twin crystals of marcasite. Decomposition : the cubes are most- 

 ly stained by a bright orange-colored tarnish, in part yellowish 

 or brownish, and decompose like the inner layer, No. 20, in a 

 damp atmosphere. 



No. 22. Pyrite. Central City, Colorado. An aggregate of 

 sharply defined striated cubes, pale brass-yellow and brilliant, 

 with slightly curved faces produced by oscillation with the 

 pyritohedron ; attached to crystallized quartz, sphalerite, and 

 siderite. Decomposition : an iridescent, bronze-colored tarnish 

 is common, even on fragments after a few months' exposure to 

 dry air ; also abundant films of whitish vitriol with strong styp- 

 tic taste, which was found to contain more ferric than ferrous 

 sulphate. 



No. 23. Pyrite. Morrisania, New York City. Very light 

 yellow granules of irregular form ; strewn thickly in thin par- 

 allel seams through a white crystalline dolomyte. Occasionally 

 an imperfect cube can be detected under the loup, ap2:)arently 

 with slio-ht octahedral modifications of its solid ans^les. Local- 

 it}', 145th street and St. Ann's Avenue. Two lots of this 

 pyrite were prepared for determination of specific gravity. 

 Decomposition ; a bright yellow iridescence is general, and in 

 many seams all the granules are deeply stained by or altered 

 into a reddish brown iron-ochre. 



No. 24. Turgite, after j^yrite. New York City. Small sharp- 

 ly defined, reddish black polished cubes, pseudomorphous in 

 turgite after pyrite ; groups upon fissures in an oligoclase-gneiss. 

 The edges of the cubes are often modified by faces of the pyrito- 



