Decomposition of Iron Pyrites. 375 



Grains of brass-yellow pyrite were enclosed in this ore, and 

 " when the pyrrhotine fell to powder by the action of the at- 

 mosphere, these particles were quite unaffected, and were seen 

 to be so many irregular spheres of cupriferous iron pyrites which 

 externally retained still a sort of skin or crust of pyrrhotine, 

 but, on breaking, at once showed that the mass possessed the 

 bright brass-yellow color characteristic of pyrites." 



A specimen of crystallized pyrrhotite in my own possession, 

 from Elizabethtown, P. Ont., Canada, exhibits a brilliant iri- 

 descent tarnish, but no farther indication of decomposition. 



It appears difficult to avoid the belief that we have, in these 

 conflicting observations concerning the stability of pyrrhotite, 

 results merely corresponding to the varying chemical constitu- 

 tion of the mineral; and that the indications of stability have 

 probably accompanied the increased proportion of sulphur, or 

 rather of the higher combination, iron disulphide — those of in- 

 stability, the predominance of iron protosulphide and nickel 

 sulphide. It appears certain that both stable and unstable 

 varieties exist. 



Pykite. 



Synonyms. — Schwefelkies, eisenkies, pyrite jaune, pyrite mar- 

 tiale, iron pyrites, cubic pyrites, mundic (term used in mines 

 of Cornwall), sulphur (term used in coal mines, clay pits, etc.), 

 iron (term used in marble quarries), coal brasses (term used in 

 the manufactories of sulphuric acid). 



Pyrite usually presents itself in opaque and brittle crystals of 

 yellowish color, brass-yellow when pure, of splendent lustre, and 

 possessing cubical, octahedral and other isometric forms ; also 

 in scales, seams, and granular nodules, often with radiated 

 structure, and commonly stained by reddish-brown films, or 

 even coated or penetrated by ochreous crusts of the same color. 



The best analyses of crystals or pure material, and determina- 

 tions of density, have been gathered into the following table, 

 in which those percentage figures for sulphur, evidently ob- 

 tained merely by deduction from 100, are enclosed in brackets. 

 Although the composition closely approximates that of iron di- 

 sulphide, it is probable that traces of impurities are constantly 

 present, e. g., gold, copper, silver, nickel, cobalt, zinc, lead, tin, 

 thallium, selenium, and arsenic ; these must exert more or less 

 effect upon the density. 



