194 Decomposition of Iron Pyrites. 



with pretty sharp outlines and rough faces, shining in part, 

 covered by limonite ; light brass-yellow and brilliant on frac- 

 ture ; imbedded in a feldspathic veinstone. Decomposition : a 

 thin shining coating of reddish-brown hepatic limonite. 



Nodule, Keyport, New Jersey. A flattened nodule, seamed 

 by radiating cracks, consisting of a fine-grained mass of pyrite, 

 coated by cubes. The crystalline outer crust and granular 

 core were separately examined. 



No. 73. (Outer crust). Marcasitic pyrite. Brilliant pale 

 brass-yellow cubes, with scaley structure, and angles modified 

 by planes of the octahedron and sometimes of the pyritohedron ; 

 grayish white, bright and uneven on fracture, with many narrow 

 shining columnar faces which are yellowish to iridescent by tar- 

 nish. Soft to the file, with brownish black streak, inclining to 

 greenish. Decomposition: a very slight yellowish tarnish, 

 rarely iridescent, with delicate incrustations of creamy white 

 gypsum. 



No. 74. (Granular core). Marcasitic pyrite. A finely gran- 

 ular cellular mass made up of minute yellowish particles. Un- 

 der the microscope, these appear as very irregular grains and 

 wrinkled films, mostly yellow and bright, nearly tin-white on 

 fracture : many grains appear striated, apparently flattened 

 cubes with octahedral modifications. Decomposition : abundant 

 particles of a white- vitriol, ferrous sulphate, throughout the 

 interstices. 



No. 75. Marcasitic pyrite. Virginia. A loose ochreous 

 brownish yellow sand, made up of pyrite-grains, 2 to 3 mm. in 

 diameter, readily brightened by digestion in acid. These com- 

 monly display the form of imperfect cubes, often hollow, and 

 modified by the octahedron ; pale brass-yellow and splendent on 

 fracture. Decomposition : a soft brownish yellow limonite- 

 ochre. 



No. 76. Marcasitic pyrite. Kelly Mine, Fauquier County, 

 Virginia. Striated cubes, modified by the pyritohedron, very 

 pale yellowish white and splendent on fracture ; in a coarse 

 quartz-veinstone. Decomposition: much intermixed with and 

 penetrated by turgite-ochre. 



No. 77. Marcasitic pyrite. Walton Gold-mine, Louisa 

 County, Virginia. A granular mass, pale brass-yellow and 



