Mineral Sulphides of Iron. 



179 



site. Of course, the method only applies where the substance 

 contains no other reducing agent than pyrite and marcasite. 

 If, therefore, we had present another crystal form of FeS 2 , the 

 results would be unreliable. Although both pyrite and mar- 

 casite have been repeatedly detected by the microscope in the 

 synthetic products, no evidence of another form has come to 

 light. It is possible that in some instances (which will be 

 pointed out in the proper place) amorphous FeS 2 may 

 have been present. In most cases, however, the crystalline 

 structure was so apparent even to the unaided eye that the 

 presence of any amorphous material was quite unlikely, and 

 this conclusion was only confirmed by the microscopic study. 



Marcasite. — Marcasite is the principal product when hydro- 

 gen sulphide acts directly on ferric sulphate at 200°. 



Exp. 1-5 g. JSTH 4 Fe(S0 4 ) 2 .12H 2 in 100 cc water saturated 

 at room temperature with hydrogen sulphide, was sealed up and 

 heated in a bomb for several days at 200°. To insure a suffi- 

 cient quantity of the product for experiment, three tubes were 

 thus heated under the same conditions. The product was 

 removed, ground fine, purified, and the oxidation number 

 determined. It was found to be 23*6, corresponding to about 

 43 per cent pyrite. 



Influence of free acid on the proportion of marcasite. — The 

 equation H 2 S -f Fe 2 (S0 4 ) 3 = S + 2FeS0 4 + H 2 S0 4 shows that 

 sulphuric acid is a product of the reaction in which the marca- 

 site forms, and its concentration evidently increases as the 

 reaction proceeds. It was, therefore, a plausible hypothesis 

 that the concentration of acid influences the crystal form ; it 

 seemed possible that the pyrite might have formed in the 

 earlier stages of the process, when the acid was weaker. If 

 this view were correct, a greater initial concentration of acid 

 should result in more marcasite. The hypothesis is proved 

 correct as shown by the results collated in the adjoining table. 



Table I. 

 The effect of free H 2 S0 4 on the formation of marcasite at 200°. 





Taken 





Found 



NH 4 Fe(S0 4 ) 2 .12 H 2 



Water saturated 

 with H 2 S 



Free H 2 S0 4 * 



Oxidation 

 number 



Per cent 

 pyrite 



5g. 

 Sg- 



100 cc 

 100 cc 

 100 cc 

 100 cc 



50 g. 

 0-57 g. 

 0-78 g. 

 1-18 g. 



23-6 

 18-9 

 17-0 

 16*5 



43' 

 25' 

 10-0 

 7'5 



* This includes the acid formed in reduction of the ferric iron by hydro- 

 gen sulphide. 



