FORMATION OF HEMATITE. 843 



contained a small amount of iron carbonate. This carbonate has a history 

 identical with that of the iron of the iron-bearing carbonates, discussed on 

 pages 824-829. Locally the iron carbonate of the limestones has been 

 picked up by the percolating waters. At various places these waters have 

 been converged at localities favorably situated. At such localities the iron 

 is precipitated from the carbonate according to the reaction: 



4FeC0 3 +20+3H 2 0=2Fe 2 3 .3H 2 0+4C0 2 . 



Often simultaneously with this process calcium carbonate has been dis- 

 solved. Thus, at the surface of a limestone formation there may be pro- 

 duced a layer of limonite which passes downward into the limestone. 



In still other cases the iron of limonites associated with limestones 

 has been in large part derived from adjacent formations. In such cases 

 the association is, in part at least, a consequence of the ready solution of 

 the limestone, which makes it easy for the substitution to take place. Such 

 substitution is likely to occur where underground waters from different 

 sources unite, as, for instance, where the limestones are fractured, so as to 

 be open and porous; where there are impervious basements, etc. 



HEMATITE. 



Hematite occurs in much larger masses than limonite. The hematite 

 is not wholly anhydrous, but contains a variable amount of water. For 

 instance, in the Mesabi hematite, the most extensive deposits of this oxide 

 known, the combined water varies from 2.09 to 8.23 per cent." Thus, there 

 are all gradations between limonite and hematite, and the majority of the 

 great soft iron-ore deposits contain limonite, hematite, the intermediate 

 oxides, and various combinations of them. The more extensive iron-ore 

 deposits of the United States, which are largely hematite, are those of the 

 Algonkian and Archean of the Lake Superior region and the Clinton ore 

 horizon of the Silurian. These masses of limonite and hematite were partly 

 deposited in place as original sediments, but to a larger extent are due to 

 subsequent segregations. 



The limonitic hematites which occur as original sediments in place are 

 identical in their development with the oxides of iron which are the original 

 source of the iron carbonates. (See pp. 824-826.) In summary, iron salts, 



"Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. S. Geo!. Survey, vol. 43, 

 1903, pp. 214-217. 



