364 THE JOURNAL OF GEOLOGY. 



The sulphides of iron and manganese differ very much in 

 their nature and mode of occurrence. Iron is frequently 

 deposited as sulphide, but manganese rarely occurs in that form, 

 and when it does it is always in very small quantities. Iron 

 forms several sulphides i-n nature: pyrite (FeSg), marcasite 

 (FeSg),^ pyrrhotite (Fe^^S^g), troilite (FeS) and -numerous 

 other more complex compounds unnecessary to enumerate here. 

 Pyrite is the commonest form of iron sulphide, and occurs in 

 rocks of all ages, from Archean to Recent. It is formed in 

 nature by the action of soluble sulphides or sulphuretted hydro- 

 gen on soluble salts of iron, and also by the reduction of 

 sulphate of iron by organic matter or other reducing agents. 

 Manganese forms two^ sulphides, alabandite (MnS) and hauerite 

 (MnSg). Both minerals are very rare, and so unstable that 

 they rapidly oxidize on exposure. Alabandite is the less rare 

 form, and usually occurs as a subordinate constituent of certain 

 metalliferous veins or allied deposits. 



Though the sulphides of manganese are easily oxidized, they 

 are not so unstable that, had they ever been formed in consider- 

 able quantities in sedimentary deposits, they would, even at con- 

 siderable depths, have left no trace of their former presence. 

 Moreover, the sulphide of manganese, as produced artificially, ^ is 

 soluble in certain organic acids, notably acetic, and, as the con- 

 ditions for the deposition of sulphides of metals in sedimentary 

 deposits generally require the presence of organic matter, it is 

 not improbable that some of the acids given off by such matter 

 would be capable of dissolving sulphide of manganese. Here, 

 then, is one reason why manganese might not be deposited as 

 sulphide under some conditions which would cause the precipi- 

 tation of sulphide of iron. Moreover, the artificial formation of 

 sulphide of manganese (alabandite) in the laboratory is brought 



^ Marcasite has the same composition as pyrite, but differs in crystalline form. 



^ Manganese also occurs in the mineral youngite, which contains lead, zinc, iron, 

 manganese and sulphur, but the mineral is considered of doubtful homogeneity. (See 

 System of Mineralogy, E. S. Dana, 1892). 



3 When manganese is precipitated artificially as sulphide it is usually in the form of 

 the monosulphide (MnS), in either a hydrous or an anhydrous form. 



