Deconqjosition of Iron Pyrites. 169 



strings of metallic iron, corresponding to tlie veinlets visible to 

 the eye, often seen within the margins of the troilite-nodnles. 

 Therefore it wonld seem unadvisable, as already suggested, to 

 consider at all this abnormal form of iron-sulphide in any ref- 

 erence' to the densities of the kinds of iron-pyrites produced on 

 (>ur own i)lanet. 



B. — Marcasitk. 



The following specimens of this mineral were carefully 

 sUidied, and their densities determined with all precautions to 

 anticipate oxidation. 



No. 1. Marcasite. Cumberland, England. Hollow incrus- 

 tation-})seudomorplis after barite, implanted on a group of 

 barite-crystals. The crusts are grayish white to tin-white and 

 splendent on fracture, with surfaces drusy with pseudo-octahe- 

 drons or rhombic pyramids, whose smooth, rectangular, termi- 

 nal faces (the basal pinacoid, OP) project but slightly ; a few 

 minute rhombic prisms also occur, and some hexagonal though 

 apparently triangular twins. Under the microscope, no impur- 

 ity whatever was seen, to account for the high density obtained. 

 The specimen, as received, was mislabelled '^pyrite." 



Evideiices of Decomposition : a copper-red tarnish with rather 

 dull iridescence. 



No. 2. Marcasite. Canterbury, England. A rounded bronze 

 yellow nodule, with mammillary surface, exactly like No. 5, 

 with the apices of the square pyramids all rounded. From 

 the Gault formation. Strikes fire readily with steel. Fresh frac- 

 ture, tin -white, with yellow iridescence upon the faces of the 

 bladed plates radiating from the nucleus ; this implies incipient 

 decomposition throughout the apparently compact nodule. 

 Decomposition : a rather dull bronze tarnish over the exterior 

 surface. 



No. 3. Marcasite. Nova Scotia. A spherical nodule of 

 bright orange-yellow tinge, imbedded in grayish clay. The sur- 

 face is drusy with truncated rhombic pyramids, apparently 

 nearly square, slightly projecting, as in No. 1, with composite 

 six-sided plates visible on their sloping faces, as well as a few 

 minute implanted rhombic prisms. Fresh fracture, grayish 



» Rammelsberg, Pogg. Ann., (1864) CXXI, 370. 



