JJecomposiiiO?i of Iron Pyrites. 175 



Ko. 28. (Nucleus). Marcasite. Dull gray and exceedingly 

 fine-grained material. Decomposition: dull iridescent yellowish 

 tarnish without, and, within, a little delicate efflorescence of aci- 

 cular white vitriol. 



Marcasite. Jefferson County, N. Y. Small drusy flakes, 

 grains and particles, sparkling in a blackish gray limestone, 

 (Trenton group). On digestion of the rock in dilute hydro- 

 chloric acid, a little hydrogen disulphide was evolved and a 

 considerable insoluble residue was left, consisting of these bright 

 grains, many particles of white quartz, and a large amount of 

 carbon in fine dull black powder. The following two materials 

 were selected for examination. 



No. 29. (Crystalline flakes). Marcasite. Glittering plates 

 of irregular form and size and with drusy surfaces, mixed with 

 slender brilliant needles and fine glittering powder ; tin-white 

 and brilliant, both on the acid-washed surfaces and on fresh 

 fracture. An abundance of quartz particles becomes visible, un- 

 der a low power of microscope, adhering to and imbedded in the 

 crusts. To their presence and, probably, that of enclosed parti- 

 cles of the amorphous carbon, we may attribute the low specific 

 gravities obtained from this material (4.51 — 4.58), like that of 

 pyrrhotite. The preliminary digestion in acid ensured the re- 

 moval of an original trace of the latter mineral, and not a 

 ])article of the powder was found to be attracted by the magnet. 

 Decomposition : on exposure to the ai r, a few delicate white 

 needles of vitriol were detected under the microscope, with mi- 

 nute white grains of fibrous radial structure. 



No. 30. (Selected crystals). Marcasite. Brilliant grayish 

 w^hite crystals, mostly twins of the usual type, deeply striated, 

 and sometimes triple. Under the microscope, there appear also 

 thin six-sided plates with striated edges and high lustre, much 

 resembling the hexagonal plates of pyrrhotite (e. g., No. 2 from 

 Schneeberg) ; these may be pseudomorphous in marcasite, after 

 pyrrhotite. 



No. 31. Marcasite. Schneeberg, Saxony. A very fine-grained, 

 glittering, crypto-crystalline mass, grayish white and brilliant on 

 fracture, made up of striated wedge-shaped twins and rhombs of 

 marcasite, with particles of grayish quartz, red garnet and scales 

 of a colorless mica. A few slightly magnetic particles were 



