172 ALLEN: ORE DEPOSITION 



occurs on coal and sometimes associated with asphalt has led 

 to the conclusion that the ferrous sulphate has been reduced in 

 some way by carbonaceous matter to FeS 2 . A disulphide ob- 

 viously could not be formed by simple reduction, but a more 

 complicated process in which ferrous carbonate would be one of 

 the products may be conceived of. The following equation in a 

 very imperfect way conveys this notion: 



7 C + 4 FeS0 4 = 2 FeS 2 + 2 FeC0 3 + 5 C0 2 



Omitting details, it may be stated that experiments which have 

 been directed toward the formation of pyrite and marcasite thru 

 the agency of carbonaceous matter have been entirely without 

 result. A possibility of synthesis is suggested by the fact that 

 pyrite and marcasite are very commonly associated with other 

 sulphides of a simpler nature, such as sphalerite and galena which 

 are directly precipitated by hydrogen sulphide. Experiments 

 have in fact shown that both pyrite and marcasite may be ob- 

 tained by the action of hydrogen sulphide on ferric sulphate 

 solutions. It is well-known that ferric sulphate is directly reduced 

 by hydrogen sulphide to ferrous sulphate with a simultaneous 

 precipitation of sulphur, but it had not previously been noticed 

 that the further action of hydrogen sulphide and sulphur on 

 ferrous sulphate slowly gives rise to a dark precipitate having 

 the composition FeS 2 . This reaction, which we may represent 

 by the equation 



FeS0 4 + H 2 S + S = FeS 2 + H 2 S0 4 



takes place even at ordinary temperature, though quite slowly. 

 A microscopic examination of product proves that it is distinctly 

 crystalline, tho the crystals are minute. If the reaction is allowed 

 to go on at a sufficiently high temperature in a sealed glass tube, 

 say 200° C, the crystals grow large enough to measure, and 

 it has been found that they are partly marcasite — a mineral 

 which has not previously been made artificially. A method was 

 devised by H. N. Stokes for the analysis of mixtures of natural 

 pyrite and marcasite which we have used to great advantage 

 on the laboratory products. It consists in the oxidation of the 



