^00 Decomposition of Iron Pyrites. 



fracture ; occupying the interstices between crystals of sphaler- 

 ite. Deco7nposition : slight brass-yellow tarnish. 



No. 105. Marcasitic ])yrite. Schoharie, New York. Small 

 reddened groups of aggregated crystals, pale yellowish white and 

 granular on fracture, or grayish Avhite with minute yellowish 

 spots ; imbedded in clay. The crystals consist of sharp cubes, 

 pyritohedra, and striated cubes with curved faces produced by 

 oscillation between the two forms. Decompositio7i : bright 

 brownish red tarnish, or blue iridescence, on most of the faces. 



No. 106. Marcasitic pyrite. Bastrop, Bastrop County, 

 Texas. Dull and pale brass-yellow octahedra, rarely bright, 

 very pale yellowish white and brilliant on fracture ; upon black 

 granular crystalline hematite. Decomposition : mostly covered 

 by or converted entirely into reddish brown hepatic limonite, 

 which is also scattered over the specimen in rusty crusts. 



No. 107. Pyrite. St. Creac, Pyrenees. Feebly shining 

 yellowish cubes, with faces finely striated or roughened and 

 slightly curved ; imbedded in a black roofing-slate. Fracture, 

 pale brass-yellow, splendent, often conchoidal. Streak, brownish 

 black. Under a lens, minute black particles are visible in abun- 

 dance through the grains of pyrite, very distinct under a com- 

 pound microscope — apparently included particles of slate. The 

 presence of this impurity accounts for the low density, as well 

 as for a peculiar granular and jDitted condition of the surface. 

 Decomposition : very slight yellowish tarnish, sometimes slight- 

 ly iridescent 



No. 108. Pyrite. Gap Mine, Lancaster County, Pennsyl- 

 vania. A thin drusy crust, finely fibrous and very minutely cel- 

 lular, with fine radial crystallization, pale yellowish white and 

 bright on fracture ; attached to a similar crust of fibrous miller- 

 ite. Under a sufiicient magnifying power, the drusy surface 

 exhibits minute geodes of microscopic yellow octahedra, sharp 

 and unmodified. Decomposition : rapidly disintegrates, even in 

 the dry air of a cabinet, with fibres falling apart ; the fresh frac- 

 ture soon assumes a yellowish iridescence and coppery tarnish, 

 with a minute efflorescence of white vitriol. 



No. 109. Marcasitic pyrite. Liskeard, Cornwall, England. 

 Clustered and composite yellowish cubes, sharply defined and 

 brilliant, often with very minute modifications of angles and 



