GEOPHYSICAL LABORATORY.* 



Arthur L. Day, Director. 

 THE IRON OXIDES. 



The great importance of the iron oxides and sihcates as components 

 of the common rock-forming minerals was pointed out in the annual 

 report for 1905, and work was begun on ferrous silicate in that year.^ 

 But difficulties were encountered in obtaining data that could be 

 interpreted, and the problem was held in abeyance while the more 

 easily accessible lime and magnesia silicates were being studied in 

 detail. Two years ago the problem was again taken up. 



The most important fact in connection with the study of the oxides 

 and silicates of iron is that gaseous oxygen must be considered as an 

 essential component. The other common rock-forming oxides (silica, 

 almnina, magnesia, lime, and the alkalies) can be treated as members 

 of condensed systems in which the relations are practically unaffected 

 by the presence or absence of atmospheric gases. The equilibrium 

 between ferric and ferrous iron, on the other hand, can vary with every 

 change of temperature and with every change in the amount or 

 pressure of available oxygen. The relations of the oxides and silicates 

 of iron can therefore be worked out only in apparatus in which the 

 pressure of oxygen is under quantitative control. Apparatus for this 

 purpose has been built and tested and has been described in a previous 

 report.^ 



The first phase of the problem is the study of the oxides of iron. The 

 properties of ferrous sihcate, for instance, can best be worked out by 

 considering it as a compound in the three-component system sihca-iron- 

 oxygen. Therefore the two-component system iron-oxygen must first 

 be understood. This system in itself is of very great importance, 

 because metallic iron is manufactured almost exclusively from oxide 

 ores. The properties of the iron oxides are therefore of fundamental 

 importance to the iron and steel industry, and the genetic problems of 

 the iron ores, both hydrated and anhydrous, form a subject of constant 

 discussion among economic geologists. 



Ferric oxide (hematite) dissociates at high temperatures, giving off 

 oxygen and leaving a homogeneous product which may be considered 

 as a solid solution of magnetite (Fe304) in hematite (Fe203). The 

 proportion of magnetite in the product depends upon the temperature 

 and upon the oxygen pressure above the oxide. The lower the oxygen 

 pressure and the higher the temperature, the more magnetite is found 

 in the soUd solution. The reaction is strictly reversible, for magnetite 



^Year Book 4, page 228 (1905). ^^nnual report, 1915, papers (9) and (10). 



♦Situated in Washington, D. C. 



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