13G Decomposition of Iron Pyrites. 



zelius, Senft acce]')ted tlie hy})othesis of that investigator as to 

 the presence and infiaence of iron protosnlphide. However, the 

 absence of definite proof of its occun-ence in marcasite inclined 

 him to snggest an additional reason for its ready alteration, viz., 

 the morphological character of the surfaces of its crystals. The 

 evenness of those of pyrite, he maintains/ must offer far fewer 

 points of attack to the agents of decomposition, air and mois- 

 ture. However, the observations presented at the close of this 

 paper (e. g., in regard to the octahedra of pyrite at Wee- 

 hawken, etc.), indicate rather that the varieties of this mineral 

 with highly polished evenly faced crystals are peculiarly liable 

 to decomposition; while those at other localities, which are 

 strongly striated, appear to possess a crystalline constitution 

 which presents unusual resistance to decay. 



9. Forced state of aggregation of particles. Fournet' jiresents 

 the following explanation of the efflorescing property of white 

 iron pyrites. ^'I have sought in the course of this memoir to 

 establish two principal facts, to wit ; that a spontaneous ten- 

 dency to dimorphism produces the disintegration of minerals, 

 and that this is followed by chemical action .. . Pyrites have 

 two forms, the one cubic, the other prismatic ; may not the lat- 

 ter be simply an unstable form ? because, as it is well known, 

 it is very liable to efflorescence, while the former resists decom- 

 position pretty generally, nnder similar conditions. Carbonate 

 of lime crystallizes in two systems, the one rhomboidal, the 

 other prismatic. The latter, which constitutes aragonite, is so 

 unstable that it is sufficient to warm a crystal a little to have it 

 fall to powder ; and besides, in Auvergne, masses are found 

 which spontaneously disintegrate without further decomposi- 

 tion." 



10. Some peculiarity of molecular aggregation. To a cause of 

 this kind, though entirely undefined, several authors have been 

 inclined to attribute the perishability of ordinary forms of mar- 

 casite. Thus, in 1849, NicoP remarked : '' This mineral is still 



1 Kryst. Felsg., 141. 



2 Ann. Oh. etPhys., (1834), LV, 255-256. 



Mineralogy, (1849), 451, 



