468 A TREATISE ON METAMOEPH1SM. 



Of the sulphides of iron, those of marcasite, pyrite, and pyrrhotite are 

 the most important. The oxidation of the iron of these sulphides may be 

 accomplished by oxygen alone, by oxygen and moisture, and finally by 

 oxygen, moisture, and bacteria. With oxygen alone the process is slow; 

 with oxygen and moisture together it is rapid-; but oxygen, moisture, and 

 bacteria together furnish the most favorable conditions. 



"Where oxidation is the only process the compound produced is mag- 

 netite or hematite. More commonly, however, hydration occurs simulta- 

 neously with the oxidation; and under these circumstances limonite and 

 other hydrated oxides of iron are produced. So far as the oxides are formed, 

 this involves desulphidation. 



sulphur. — At the same time the iron is oxidized the sulphur united with 

 it may also be oxidized. Where the two are oxidized together and remain 

 united the result is to form iron sulphate. Frequently, however, at the 

 time the iron is oxidized the sulphur or some part of it separates as hydro- 

 sulphuric, sulphurous, or sulphuric acid. The various reactions resulting 

 in these compounds are given on pages 214-216, and need not here be 

 repeated. By reference to the reactions there written it will be seen that 

 certain of them, as already stated, result in the formation of hydrosulphuric 

 acid. Indeed the reactions producing hydrosulphuric acid are very 

 common during the oxidation of pyrite, marcasite, and pyrrhotite. In a 

 similar manner the oxidation of the monosulphides of the other metals may 

 result in the production of hydrosulphuric acid. 



The sulphur of the hydrosulphuric acid may be later oxidized to sul- 

 phurous or sulphuric acid. While the oxidation of hydrosulphuric acid ma} T ' 

 be accomplished by water and oxygen without the assistance of bacteria, 

 often bacteria are the inciting cause of the change. Where hydrosulphuric 

 acid is abundant the hydrogen only is first oxidized and the sulphur stored 

 in the cells of the bacteria, according to the reaction: 



H,S+0=H 2 0+S 



But finally the stored sulphur is oxidized by the bacteria to sulphuric 

 acid, which reacts upon the bases present and forms sulphates. 11 



In the transformation of the sulphides the volume change on account 

 of desulphidation produces considerable diminution in volume, provided 



«Lafar, pit., pp. 370-374. 



