138 CARNEGIE INSTITUTION OF AVASHINGTON. 



readily takes up oxj-gen at temperatures above 300° until the equi- 

 librium proportion at the temperature in question is attained. Mag- 

 netite is therefore a chemically unstable mineral under atmospheric 

 conditions, although it may remain unoxidized for long periods, just 

 as the physically unstable fonns of silica (tridymite, cristobalite) 

 and of calcium carbonate (aragonite) persist unchanged at ordinary 

 temperatures after they have once been fomied. 



The dissociation pressure of magnetite is extremely low, yet it also 

 can be dissociated at high temperatures under low oxygen pressure. 

 The product is intermediate in composition between Fe304 and FeO, 

 but the exact relations are not yet determined. The properties of 

 ferrous oxide also are as yet little known. Most of the pubUshed 

 methods for preparing FeO yield only a mixture of metallic iron and an 

 oxide intemiediate in composition between FeO and Fe304. The 

 opacity of all these products deprives us of the valuable aid usually 

 obtained from the petrographic microscope. 



Practically all natural iron oxides, hydrated or anhydrous, contain 

 a determinable amount of ferrous iron. The proportion ranges all the 

 way from a few hundredths of 1 per cent of FeO, in some hematites, up 

 to the percentage in pure magnetite. The amount and manner of 

 combination of this ferrous iron give important indications of the origin 

 and history of the oxide, as may be readily seen from chemical consid- 

 erations. Ores formed by the simple hydrolysis at low temperatures 

 of ferric compounds will contain practically no ferrous iron. Hematites 

 of magmatic origin may contain several per cent of FeO. A particu- 

 larly interesting feature of the latter is the occasional zonal distribu- 

 tion of the ferrous iron occurring, for instance, in hematite crystals from 

 the ore deposits of Elba. This zonal growth shows clearly that the 

 temperature or the oxygen concentration, or both, were changing con- 

 tinuously in one direction during the deposition of these crystals. 



Ferric oxide exhibits a reversible thennal inversion-point at about 

 680°, corresponding to a magnetic inversion which was already known. 

 Analogous magnetic inversions occur in metaUic iron and in magnetite, 

 although at difTerent temperatures and with a very much greater 

 change in magnetic susceptibility. Some intimate atomic relationship 

 undoubtedly exists among these inversions in iron and its oxides, 

 from which increased light on the atomic constitution of the oxides 

 and the nature of chemical valence may some day be obtained. 



The magnetic properties of the oxides offer a most interesting field 

 for investigation from the standpoint both of geology and of pure 

 physics. The attraction exerted upon a given weight of oxide by a 

 magnet is almost directly proportional to the percentage of FeO in 

 solid solution through the range of compositions from Fe203 to Fe304. 

 Ferrous oxide not in solid solution in the oxide, however, as in some 



