Decomposition of Iron Pyrites. 129 



B. — Explanations of the Tendency to Decomposition. 



Various theories will be now briefly considered, which have 

 been advanced to account foi- this singular difference of stabil- 

 ity, not only between ordinary marcasite and other pyrites, but 

 also between the varieties of a single mineral of definite chem- 

 ical composition, pyrite. 



1. Presence of some foreign impurity. In one of the earliest 

 discussions of the subject, J. F. Henkels remarks,^ " it is a fact 

 that the nodular pyrites virriolesces more easily than the angu- 

 lar, likewise the radiul than the laminated ; I find also the causes 

 in copper and arsenic, whereof the nodular, as also the radial, 

 are wont to be free." AVith these two agencies, however, he also 

 associates the texture of the mineral, with its accompanying den- 

 sity, and points out that one or more of these three conditions 

 may be concerned in the difference of stability in one case or an- 

 other, quoting the maxim, unius rei plures possimt esse causes. 

 Still later, Werner expressed the suspicion that the presence of 

 arsenic, and Berzelius, that of manganese, etc., might be con- 

 nected with the diffei-ences of decomposition in pyrites. The en- 

 tire absence of these substances, however, from some of the most 

 unstable specimens, and their occasional presence in pyrite of 

 stable character, controvert this view. The instances are exceed- 

 ingly rare in which such a connection has been established, e. g., 

 that of the pyrite of Cornwall, Penn., already referred to. 



2. Presence of free sulphur. Stromeyer long ago suggested,^ 

 ^' It may perhaps be yet ascertained that pyrites may not be the 

 true iron sulphide in maxiino.'' i. e., with the formula, Fe S", 

 '^ but a combination of the same with sulphur hydrate." The 

 same view was adopted by others, but, it is sufficient to say 

 it has not been confirmed by the most careful analysis. 



3. Inferior proportion of comii7ied sulphur. This view of 

 Proust, to account for the more ready alteration of certain S2:>ec- 

 imens of pyrites, is stated beyond in the words of Hatchett, but 

 remains equally unsupported by later investigation. 



' Pyritologia, Leipzig, (1754). 782-786. 



■^ Gilb. Ann. d. Phys., (1814), XLVIII. 189. 



