Mineral Sulphides of J r on. 171 



insoluble in water that some statements* regarding their solu- 

 bility, found in the literature, could only have been made on 

 the basis of experiments in which atmospheric oxygen was 

 not excluded. In all these instances the two minerals behave 

 much alike, though marcasite is always more soluble in any 

 medium and is more readily changed by oxidizing agents. 



Formation of iron disulphide in nature. — Geologists appear 

 pretty well agreed that pyrite sometimes crystallizes direct from 

 rock magmas. Pyrite, however, in the great majority of cases, 

 and marcasite in all cases, crystallize from water solutions, 

 though the nature of the process is purely a matter of conjec- 

 ture. Here a general line of division should be made between 

 the products of hot and the products of cold solutions. The 

 pyrite of deep veins, metamorphic contacts and hot springs, as 

 well as magmas, has been formed by hot solutions, and such solu- 

 tions never contain strong mineral acids, but are generally if 

 not always alkaline. The pyrite and marcasite of surface veins, 

 on the other hand, are formed from cold solutions which often 

 contain considerable sulphuric acid. We shall find cogent 

 reasons for the conclusion that the chemistry of these two proc- 

 esses is similar, but first let us consider the formation of iron 

 disulphide from surface solutions. Here the geologic hypothe- 

 sis is that both minerals have been formed by the "reduction " 

 of ferrous sulphate through the agency of organic matter, and 

 indeed, the frequent occurrence of pyrite in coal and its occa- 

 sional formation on wood gives plausibility to this view. It is 

 plain that the soluble sulphate of iron could not be changed by 

 simple reduction to the disulphide, though one might imagine 

 such a reaction as the following equation imperfectly repre- 

 sents * 



K (vegetable matter) +4FeS0 4 =2FeCO, + 2FeS a + 5CO j ,. 

 Some experiments have been tried in this laboratory in the 

 hope of u reducing " ferrous sulphate with organic matter, but 

 the results have not been promising. The action of starch and 

 glucose on aqueous solutions at 300° was either slight or nil. 

 On the other hand, the possibilities of hydrogen sulphide are 

 suggestive. Pyrite and marcasite are very often found with 

 simpler sulphides, — those of lead and zinc for example, which 

 may be easily formed by the action of hydrogen sulphide ; and 

 furthermore, hydrogen sulphide is a substance widely distrib- 

 uted in nature. Without denying that carbonaceous substances 

 may in some instances be directly active in the formation of 

 pyrite and marcasite, we will proceed to show that both min- 

 erals may be formed through the agency of hydrogen sulphide, 

 and under conditions which doubtless prevail commonly in 

 nature. 



*See Doelter, Tschermak's Min. Petr. Mitt., N. F. xi, 322, 1890 ; Neues 

 Jahrb., ii, 273, 1894. 



