390 Decomposition of Iron Pyrites. 



Only analyses and specific gravity determinations upon selected 

 rhombic crystals of pure marcasite can have any definite value, 

 and not a single investigation of that kind is on record ; most 

 of the analysts have not specified the exact kind of material 

 used, nor whether the density was determined on the coarse 

 powder. So that we have yet no evidence of the absolute 

 chemical composition of marcasite, except, of course, that it 

 closely approximates that of pyrite (Fe S 2 ), nor of its exact 

 density. For the latter, the average of all the figures above 

 given is -4.84T. 



Weathering of Marcasite. 



The phenomena of vitriolescence, i. e., the development of 

 copperas, are best displayed in the ordinary decomposition of 

 this mineral. 



Senft states, in regard to its alteration : " In moist air usu- 

 ally weathering very rapidly, and covering itself with an earthy 

 or fibrous mould-like coating of copperas; but sometimes also 

 becoming brown like pyrite, and then resisting the farther 

 attack of moist air." 55 



According to Gmelin : ''Most yellow iron pyrites, and like- 

 wise the white variety when well crystallized, remains unaltered 

 in moist air." 56 



T. Egleston 57 refers to the ready oxidation of marcasite, and 

 also confirms the last author in reference to a more durable form : 

 . " It decomposes very easily in the air, and forms sulphate of 

 iron. In order to preserve it in collections, it must generally 

 be coated with varnish. Sometimes, though rarely, the product 

 of decomposition is hepatic pyrites, as is the case with the balls 

 that occur in the chalk. A variety is sometimes found which 

 does not decompose." 



The alteration of marcasite into limonite, above referred to, 

 has been observed in specimens from Upper Silesia, Derbyshire, 



55 DieKrystall. Felsg., 140. 



56 Hand-book of Chem., V, 234. 



51 Lectures on Min. (1871), Pt. II, 123 



