128 Decompoi^itioii of Iron Pyrites. 



suffers decom})osition through the action of air and moisture ; 

 although this occurs with the latter only under more favorable 

 conditions. Pyrite, in compact rock and in large particles, re- 

 sists almost all decomposition. If the same mineral, on the 

 other hand, occurs in fine distribution, e. g., in alum-slate, 

 which, at the same time, on account of its loose texture, cannot 

 reject infiltrating moisture, then the decomposition takes i)lace 

 most thoroughly and with ])roportionate rapidity." Menes 

 also,* after his examination of the massive pyritous deposits of 



France, concludes : '• The yellow iron pyrites occurs more 



particuhirly in the crystalline rocks aud preserves itself very well 



in the air, while the white iron pyrites is always found in 



sedimentary deposits and readily effloresces, with a formation of 

 copperas." In the latter investigaiion by Girard and Morin,^ of 

 the French pyrites, they rem;irk, in regard to the pyrite of 

 Sain-Bel, which occurs in argillo silicious schists of Silurian 

 age : " This is sulphide of iron almost chemically pure, and this 



purity ]n'e vails in almost all pjirts of the mass This pyrites 



is very slightly altei'able ; it only oxidizes with difficulty at ordi- 

 nHry temperatures." 



This difference of stability nuiy be readily verified in any min- 

 eralogical cabinet, by an examination of the comparative condi. 

 tion of the specimens of ])yrite derived from the coal-measures, 

 lignitic shales, and clays, etc., and those from the crystalline 

 rocks of some highly metamorphic n-gion, like Elba or Colorado. 

 The former are invariably more or less crumbling and rusty, in 

 spite of complete protection, from ordinary weathering, within a 

 building and even inside of tight glass cases ; the latter remain 

 hard and brilliant, even thouo^ii they may have already been ex- 

 posed to the weather for years, while lying in the waste upon 

 the dump of some mine. 



This shai-p distinction between the varieties of pyrite is there- 

 fore unquestionable, as well as the existence of other forms 

 which appear intermediate, in all degrees, in regard to resistance 

 to natural oxidation. 



' Compt. Kend., (1867), LXIV, 867. 



■' Ann. Cb. etPhys., (1876), 5 Ser., VIII, 229. 



