170 Decomposition of Iron Pyrites. 



white, with a slight yellowish tinge caused by incipient tarnish 

 on the faces of intersecting bladecl plates, sometimes drasy with 

 microscopic crystals of the same mineral. 



Decomposition : a slight orange-yellow tarnish. 



No. 4. Marcasite. Bohemia. A nodule made np of clustered 

 groups of crystals encrusting a fibrous blackish nucleus. Com- 

 l^osite platy rhombic pyramids, sometimes Avith the rectangular 

 terminal faces (OP) ; also in striated twins. On fracture, 

 grayish white and brilliant. The crystals only were used for 

 the specific gravity determination. 



Decomposition : on the crystals, a dull blackish film and 

 bluish iridescence ; on the nucleus, some efflorescence of white 

 and yellow vitriol, chiefly ferrous sulphate, which corrodes the 

 specimen-label. 



No. 5. Marcasite. Folkestone, England. An elongated 

 rounded nodule, with finely granular nucleus. The outer crust 

 is grayish white and brilliant on fracture, coarsely radial with 

 thin platy structure. Surface made up of thin rhombic plates 

 of the usual six-sided form, in nearly square mammillary aggre- 

 gations ; also twinned triangular plates. The terminal faces 

 (OP) are nearly square. From the Chalk. Decomposition: 

 bronze yellow tarnish. 



No. 6. Marcasite. Belgium. Very ij-regulai* nodule with 

 rough prongs and projections. Grayish white and brilliant on 

 fracture. Surface drusy with composite quadrilateral pyramids, 

 made up of scaly, ill-developed i)]ates, sometimes octagonal ; 

 terminal face (OP) broad. Strikes fire readily with steel. De- 

 composition : a fresh fracture soon becomes yellowish. Outer 

 surface covered by a dull brownish yellow and brownish black 

 film, inky to the taste, with traces of an efflorescence of white 

 vitriol. 



No. 7. Marcasite. Hannover, Germany. Tooth-like rhombic 

 pyramids of the type of Galena, 111., and twinned forms, with solid 

 angles bevelled and faces slightly sti-iated. Grayish white and 

 brilliant on fracture. Decomposition : dull blackish gray tar- 

 nish, sometimes with a blue iridescence. 



No. 8. Marcasite. Marsden's Diggings, Galena, Illinois. A 

 coarsely nodular crust upon sphalerite. Grayish white and 

 brilliant on fracture. Surface studded with tooth-like rhombic 



