56 sosman: problems of the oxides of iron 



stand ferrous silicate and the minerals in which it occurs, there- 

 fore, we must go back one step further ; and study the system 

 iron-oxygen. 



THE PHASE-RULE DIAGRAM OF THE SYSTEM IRON-OXYGEN 



The logical point at which to begin a survey of the pressure- 

 temperature-concentration relations in the two-component sys- 

 tem iron-oxygen is with the pure component iron. We have 

 learned a great deal in recent years about the properties of pure 

 metallic iron, although the difficulty of preparing the metal 

 quite free from its usual impurities of carbon, hydrogen, sulphur, 

 phosphorus, manganese, and silicon, has delayed the determi- 

 nation of its elementary constants. 



But when we add oxygen, the second component, to the sys- 

 tem we come into a little-known region which extends as far as 

 the compound Fe 3 4 . The solubility of oxygen in solid iron 

 is certainly not large, but has not been accurately determined. 

 Austin 2 states that the oxygen found by analysis of iron bars 

 does not exceed 0.29 per cent, and that the most of this seems 

 to be in the form of oxide films in mechanical admixture. Pick- 

 ard 3 concludes from his analyses that 0.092 per cent of oxygen 

 is soluble in liquid iron as FeO, although a part of this may sep- 

 arate out on the solidification of the metal. It is thus evident 

 that there is no extensive solid solution of oxygen in iron. 

 But the small quantity that is present has a very large influence, 

 especially on the magnetic properties of the metal, as appears 

 from the work of Yensen. 4 



The next question is, what is the lowest oxide in equilibrium 

 with metallic iron? Ferrous oxide, FeO, has been generally 

 assumed to be the oxide lowest in oxygen, but the data concern- 

 ing it are far from satisfactory. Various methods for preparing 

 FeO are found in chemical literature. Mr. Hostetter and I 

 have tried several of these, and our experience has invariably 



2 Engineering, 100: 455. 1915. 



3 Iron Age- 98: 184-186. 1916. 



* Yensen, T. D. Univ. Illinois, Eng. Exp. Station, Bulls. 72, 77, 83. 1914- 

 1916. 



