190 Decompositi07i of Iron Pyrites. 



No. 57. Pyrite. Lee, Massachusetts. Very minute glitter- 

 ing yellow scales and particles, usually 0.1 to 1.2 mm. in length, 

 scattered in the proportion of three or four to every square cent- 

 imeter of surface, in fine white dolomytic marble ; associated 

 with particles of tremolite, phlogopite, black and brown tour- 

 maline, rutile and quartz, in the residue left on solution of sev- 

 eral pounds of the marble in acid. The crystals are mostly 

 cubes with highly polished faces, rarely showing a few striae un- 

 der the microscope ; commonly passing into pyritohedra, and 

 sometimes showing the pyramidal faces of the tetrahexahedron ; 

 pale brass-yellow and splendent on fracture. Decomposition : 

 inclining to rapid alteration, partly or completely into reddish 

 brown particles of limonite, with an orange-yellow ochreous halo 

 extending 1 to 5 mm. or more through the marble, around the 

 particle. 



No. 58. Marcasitic pyrite. Bay of Chaleur, Lower St. Law- 

 rence, Canada. A dull bronze-colored, round, compact nodule, 

 bright grayish white, faintly yellowish, on fresh fracture. The 

 exterior is covered by large smooth-faced cubes, with angles 

 broadly modified by octahedral faces — approaching cubo-octa- 

 hedra — and with surfaces marked by scaley composite aggre- 

 gations. Decomposition : the crystals are covered by a dull 

 bronze-colored tarnish ; the cavities between them, by copper- 

 red films of iron-oxide, with a slight efiiorescence of white silky 

 needles of sodium-sulphate, with incipient cracks in the nodule ; 

 the fresh fracture, by a rapid yellowish and orange-yellow tar- 

 nish, on exposure. 



No. 69. Marcasitic pyrite. Dubuque, Iowa. Thin mam- 

 millary and botryoidal drusy films, pale brass-yellow aud splen- 

 dent on fracture ; implanted on sphalerite and galenite. The 

 crystals consist of composite scaley octahedra of pyrite, covered 

 by adhering crusts of microscopic flattened coffin-shaped rhom- 

 bic plates of bronze-colored marcasite, whose presence accounts 

 ior the low specific gravity obtained. Decomposition : a general 

 dull yellow tarnish. 



No. 60. Marcasitic pyrite. Oharlemont, Massachusetts. A 

 coarsely granular, glittering yellowish mass, rather loosely ag- 

 gregated, of grains which are grayish white with spots of orange 

 and yellow tarnish, and splendent on fracture. Nearly every 



