70 sosman: problems of the oxides of iron 



are undergoing oxidation. The usual "magnetic hematite/' how- 

 ever, owes its magnetic properties to its ferrous iron content. 



A simple magnetic test, then, combined with a determination 

 of the ferrous iron, permits certain preliminary conclusions as to 

 the origin of many natural oxides of iron. But there are other 

 magnetic properties which we have not yet touched upon, which 

 may be expected to vary with the composition and constitution 

 of the oxide, such as the remanence and the coercive force, and 

 especially the variation of the magnetic properties with tempera- 

 ture. This correlation of magnetic, physical, and chemical proper- 

 ties is not a small task, but it is one for which the apparatus 

 and methods are now in good working order. 



The remanence, or permanent magnetization, offers particular 

 interest. It is well known that magnetite crystals are frequently 

 found which are very strong permanent magnets; in fact the 

 history of magnetism dates from the discovery of these "lode- 

 stones" by the ancients. Yet most natural magnetite is not 

 polarized. What causes have produced the polarization and 

 how are they related to the origin and history of the ore? 



There is time to touch only in the briefest way on the prob- 

 lems connected with other types of iron ores. The study of 

 replacement ores leads us into the still obscure problem of re- 

 placement as a geological phenomenon, ably discussed by Lind- 

 gren in a recent paper 29 but still, unfortunately, the subject 

 of more discussion than experiment. The secondary concen- 

 tration of oxide ores by circulating waters is another problem in 

 chemistry at ordinary temperatures which has not yet received 

 the attention it deserves. 



Even the origin of ores which are admittedly almost unaltered 

 is still somewhat obscure. The genesis of residual ores such 

 as those of Cuba is not so much a problem of the oxides of 

 iron, since these play rather a passive role, as of the weathering 

 of iron-bearing silicates. But the origin of certain sedimentary 

 deposits is a specific problem in the chemistry of iron, complicated, 

 however, by a new factor, the biological. It has long been 

 known that some of the higher bacteria play a part in the prc- 



29 Lindgren. W. The nature of replacement. Econ. Geol., 7:521-535. 1912. 



