Decow poiiii ion uf Iron Pyriles. 211 



3. Latent cutistitutloii of jjyriie crystals. Using therefore 

 the figure 5.01, foi* tlie specific gravity of pyrite, in the same 

 way as before, in the formula stated under marcasite, the theo- 

 retical percentage proportion of marcasite, i"»resent in the speci- 

 mens of pyrite, lias been calcuhited in the figures of the fifth 

 column of the Table. The twelve specimens (Nos. 94 to 106), 

 which contain 50 per cent, or more of marcasite, are evidently 

 paramorphs after pyrite, two of them, Nos. 97 and 100, corres- 

 ponding to specimens of pure pyrite, Nos. 10 and 46, from the 

 same localities. \\\ this series, there is even more decided evi- 

 dence, than in that of marcasite, of the influence of marcasite on 

 the physical propei'ties of the mixture, even to the paling of the 

 color. A similar lowering of density and paling of color, from the 

 suspected i)resence of marcasite, have already been pointed out 

 by others, e. g., by Kohler, in the crystals of pyrite from Gross 

 Allmerode'; the densities of crystals reported from that locality, 

 4.941 to 4.845, would imply a content of 31.92 to 77.84 percent, 

 of marcasite, in my view. 



At about the specific gravity 4.95, corresponding to a content 

 of 26 or 27 per cent, of marcasite, the crystals begin to show the 

 influence of the contained impurity by a paling of color and 

 marked tendency to decomposition, and this material may be 

 called marcasitic pyrite. It is a significant fact that the var- 

 ieties of the mineral of commercial importance, especially for 

 the manufacture of sulphuric acid, are found near or below this 

 limit, e. g., Nos. 60, 63, etc. The series of specimens of marcas- 

 itic pyrite does not become continuous, however, until the spe- 

 cific gravity descends below 4.92, corresponding to a content of 

 38 per cent, of marcasite. This point therefore seems to mark 

 the danger-limit in cases where a rapid tendency to oxidation 

 becomes objectionable, as with pyrite j^resent in building-stones, 

 roofing-slate, and coal. 



It should here be added that, although marcasite appears to 

 be the general element of instability and agent of decomposition, 

 we have evidence in this series of the accessory agency of other 

 sulphides in some cases. The action of chalcopyrite has already 

 been noted, in the case of the cupriferous pyrite of Cornwall, 



'Pogg. Ann., (1828), XIV., 91— Analysis No. 8 by Rammelsberg, Part I. 

 of tMs paper, pages 388, 389. 



