174 Decomposition of Iron Pyrites. 



-are commonly separated by finely grannlar layers, sometimes 

 one to three millimeters in thickness, tin-white and brilliant on 

 fresh fractnre. Decomposition: on the mammillary surfaces, an 

 iridescent tarnish : on the granular layers, a bronze-colored 

 tinge, with some delicate efflorescence of white Yitriol. 



No. 24. (h, Middle layer, granular crystalline). Marcasite. 

 A layer in which the concretionary lines and fibrous structure 

 are more strongly marked, and the granular layers distinctly 

 exhibit the simple six-sided rhombic plates, coffin-shaped, and 

 some triangular twins, presenting the square terminal faces. 

 Some of the abundant drusy surfaces in this layer consist of 

 composite rhombic pyramids, made up of six-sided coffin-shaped 

 plates, presenting their terminal faces (OP) in close aggrega- 

 tions, as fine parallel lines. Decomposition : as in the layer No. 

 23, and with brilliant iridescent tarnish upon the visible crystals 

 in the drusy cavities. 



(c, Outer crust). Pyrite. (See No. 12 in Table of Pyrite, 

 beyond). A coarsely fibrous or columnar layer of pyrite, pale 

 3^ellowish white and brilliant, with Sp. Gr. = 5.010. 



No. 25. Marcasite. Galena, 111. A thick crust on sphalerite, 

 light grayish white and bright on fracture. Surface covered by 

 symmetrically arranged and closely aggregated, twinned rhombic 

 crystals, often finely striated. Decomposition : a beautiful and 

 lustrous iridescence, green, yellow and light crimson. 



No. 2Q. Marcasite. Hazelgreen, Wis. Bright crusts, lining 

 cavities in calcite-veins through limestone ; grayish white and 

 brilliant on fracture. Surface covered by curved, finely striated 

 and lamellated groups of rhombic prisms, wedge-like rhombs 

 of the Joplin type, (No. 12). Decomposition : bronze-colored 

 and iridescent greenish tarnish. 



Concretionary nodule. Crow Branch Mine, Wis. Bright 

 compact drusy crusts, associated with cubes of galenite, upon a 

 fine-grained, dull gray nucleus. 



No. 27. (Outer ci'ust). Marcasite. Bright and massive 

 crypto- crystalline material, with drusy cavities, grayish white 

 and brilliant on fresh fracture. Finely striated rhombs and 

 twin crystals, and thin brilliant plates with serrated edges, a 

 miniature form of "cockscomb pyrites". Decomposition : a sul- 

 phurous odor is evolved by the material when bruised. 



