ALLEN: ORE DEPOSITION 173 



sulphides by a dilute solution of ferric sulphate. The products 

 of the reaction are ferrous sulphate, sulphur, and sulphuric acid 

 for both minerals, but the relative quantities of the products 

 are decidedly different, Only 14 per cent of the sulphur in mar- 

 casite is thus oxidized to sulphuric acid, the rest being precip- 

 itated as free sulphur, while 56 per cent of the sulphur in pyrite is 

 oxidized. It is thus possible to determine in any mixture of 

 the two sulphides the percentage of each. As was stated above, 

 the action of hydrogen sulphide on a mixture of ferrous sulphate 

 and sulphur gives in general a mixture of both pyrite and marca- 

 site, and the application of Stokes' method has proved that there 

 are two essential conditions which determine the proportions of 

 the two; these are temperature and acidity. The higher the 

 temperature during the reaction, the greater is the percentage of 

 pyrite obtained. It is also true that the lower the acidity of 

 the solution, the greater is the percentage of pyrite formed, until, 

 when the solution becomes neutral or alkaline, the product is 

 practically all pyrite. The equation on p. 172 shows that it is 

 not possible to maintain the solution neutral when hydrogen 

 sulphide acts on ferrous sulphate and sulphur, for sulphuric acid 

 is one of the products, but if we substitute suspended ferric 

 hydroxide for the ferrous salt, the solution remains neutral, if we 

 waive the question of the weakly acid nature of hydrogen sulphide 

 itself. Another way in which we can command the same essential 

 conditions is to heat pyrrhotite or ferrous sulphide with free 

 sulphur and a water solution of hydrogen sulphide which contains 

 a very little sodium bicarbonate. This solution dissolves and 

 carries over the sulphur to the pyrrhotite, thus forming FeSj in a 

 practically neutral solution and the result is pyrite. 



An alkaline solution of sodium polysulphide precipitates from 

 a ferrous salt an amorphous mixture of ferrous sulphide and 

 sulphur, but these combine slowly under the influence of heat, 

 and probably, even at the ordinary temperature, with the forma- 

 tion of pyrite. 1 These data enable us to draw some important 



1 At the lower temperatures (100°) amorphous disulphide appears to be the 

 6rst product. 



