120 Decomposition of Ivo}i Fjfvites. 



the effloi'escing \ iti-iol bj reacLion with salts of the sea-Avater, 

 the deposit of hydrated iron-oxide, and the i:)artial or complete 

 protection of many nodules from farther oxidation, by envelop- 

 ment in a limonite-crust, and by its infiltration into their pores. 

 In few cases on recoi'd can we trace a distinct connection be- 

 tween an nnusual facility of decomposition, in a variety of py- 

 rite, and the presence of another metallic sulphide : e. g., the 

 cupi-iferous varietv^ from Cornwall, Lebanon County, Penn., of 

 which.it is stated,^ " it tarnishes readily, assuming the blue tar- 

 nish of steel." But we have now to consider another class of 

 facts, of entirely different nature and general occurrence, which 

 bear on the stability of the same mineral, independent of any pe- 

 culiar local conditions. 



I. Ox THE Variation of Stability ix Pyrite. 



The general tendency of this mineral to oxidation, on ex- 

 posure to moist air, is so characteristic and so quickly apparent 

 to the most unskilled, and the varieties of pyrite possessing it 

 are of so wide and abundant oocurrence, that the existence of 

 the stable variety is not as yet familiarly known. 



A. Insicoices of Jicsis/djire to Decomposition. 



'I'hese occuirences, though well marked and brought to the 

 notice of uiMuy mineralogical students and collectoi'S, have been 

 j-arely recorded and never collated. A late reference to the sub- 

 ject has been made by Dr. J. S. XewberiT -J^ 



'• A peculiarity of this mineral is the readiness with which 

 some specimens oxidize, while others, apparently similar in all 

 other respects, remain brilliant. Few more beautiful minerals 

 ever enter a collection ; but many of the specimens of pyrite in 

 the cabinet of the School of Mines, particularly those from 

 Schohai-ie, N". Y., decomposed rapidly, absorbing oxygen and 

 water, thus forming sulphuric acid, which has destroyed labels 

 and trays, and has even cut through the bottom of the drawer 



1 Analysis Xo. 25, Part I of this paper, Ann. N. Y. Acad. Sci., (1886), 



111, 877. 

 « J. D. Dana, System of :\Iincialogy, (1883), 63. 

 ■' Tnms. X. Y. Acad. Sci., (1888), TI, 138-139. 



