178 Decomposition of Iron Pyrites. 



calculated by the usual formula, founded on the relationship 

 of the loss of weight to specific gravity : 



100— a: X 100 



_ + = . 



5.01 4.80 a 



in which x represents the percentage proportion of marcasite in 

 the specimen under triarl, and a the specific gravity of the 

 specimen. For convenience, this formula may be reduced thus : 



11451.4286 



x = — ^ 2285.71 



a 

 and in this way and by occasional interpolation the figures in the 

 two columns have been obtained, which exhibit the supposed per- 

 centage constitution in marcasite and pyrite, corresponding to the 

 specific gravity of each specimen. It is admitted that the density 

 of some samples has been probably affected by other constituents, 

 e. g., it may be, by the enclosure of a little argentite (sp. gr. 7.2) 

 and galenite (sp. gr. 7.5) in some of the samples of marcasite 

 (Nos. 8 and 9) from Galena, Illinois, etc. But it is of equal sig- 

 nificance that, in six other samples from that very locality, the 

 density falls to the lowest point (Nos. 16 to 18 and 21 to 23). 

 Again, that peculiarly brilliant, hard and stable variety of mar- 

 casite, which is found in the Chalk and Gault formations of 

 England, presents the highest densities (Nos. 2, 5, 11 and 14), 

 and entire freedom from heavy constituents ; see analysis of No. 

 11 already given. The figures shown in the analyses of marcas- 

 ite from other localities, by Rammelsberg, etc.,^ bear the same 

 testimony. It seems therefore reasonable to conclude that the 

 main disturbing element, in the variation of the density of 

 specimens of marcasite, must in general be the intermixture of 

 pyrite suggested by the similar variation in physical properties. 

 From this it will also follow that the varieties of high density at 

 the head of the table, consisting of over 50 per cent, of pyrite, 

 are all true paramorphs after marcasite. That the color of these 

 is not perceptibly affected by a yellow tinge, as in the similar 

 paramorphs described beyond under pyrite, may be due to a uni- 

 form dissemination of the molecules of pyrite in the former and 

 their partial concentration in the latter. The paramorphs, here 

 presented under marcasite, may also have mostly originated by 



1 Part T, 388, Analyses Nos. 7 to 10. 



