Decomposition of Iron ryriles. 147 



of the nioditication of cubes of j)tjle biass-yellow color and bril- 

 liant lustre, certainly consisting of crystallized pyrite. No vis- 

 ible reason for the decay was apparent in these crystals. 



From these microscopical observations, presented elsewhere^ 

 in fuller detail, we may sum np the following conclusions : 



First. In these fibrous nodules of pyrites, the outward 

 growth of the elongated cubes, of which the fibres consist, and 

 their mutual compression, have produced a condition of great 

 tension, which has facilitated their later disintegration. 



Secondly. The material is mainly composed of a diluted 

 mixture of pyrite with a paler colored and unstable impurity. 

 Through this mixture, more or less pure pyrite is diffused in al- 

 ternating films or in scattered strings and crystals of a deeper 

 yellow color than that of their matrix. 



Thirdly. The oxidation of the material has been facilitated 

 by its heterogeneous composition, by its fissured structure, and 

 by the tension among its fibres. It has progressed more rapidly 

 in the predominant paler colored mixture, has penetrated along 

 the seams between the fibres, and has then been hastened by the 

 development of the more minute fissuring, as the result of the 

 tension. 



Fourthly. The development of this system of minute fissures 

 has furnished an enormous area for the internal condensation of 

 gases and va| ors from the atmosphere, chiefly oxygen and mois- 

 ture, whic has resulted in the speedy oxidation, pitting, de- 

 cay, production of crystals of vitriol, expansion, and final disin- 

 tegration observed in such forms of })yrites. 



Fifthly. The mode of oxidation in all forms of pyrite is es- 

 sentially the same, resulting in the initial production of ferrous 

 sulphate and free sulphuric acid. By the removal, decomposi- 

 tion, or neutralization of the latter, oxidation of the ferrous salt 

 is promoted, which may be then rapidly converted into one or 

 more ferric sulphates, when freely exposed to the air. In the 

 purer forms of pyrite, subject to but slow oxidation, the results 

 of decomposition are washed away as fast as they form, and the 

 surface of the mineral retains its brightness, if the grain lies ex- 



^ The Microscopical Structure of the Iron Pyrites, Jour. N. Y. Micr. 

 Soc. (1886). 1-12. 



