RELATIONS OF OXIDATION AND DEOXIDATION. 469 



the oxide of iron is not hydrated — with magnetite, 24 to 39 per cent; but 

 in case both oxidation and hydration occur together there is increase in 

 volume of but 3 per cent in the case of pyrite and 25 per cent in the case 

 of pyrrhotite. 



GENERAL STATEMENTS. 



It might be inferred from the foregoing statements in reference to the 

 volume relations that, on the average, there is a decrease in volume in 

 oxidation rather than an increase, as one would naturally expect. In 

 support of this view it has been stated that there is generally a decrease in 

 volume in the processes of oxidation of the carbonates and sulphides, since 

 the process of oxidation simultaneously results in decarbonation or desul- 

 phidation. However, much more important than the oxidation of iron in 

 the form of carbonate and sulphide is the oxidation of ferrous iron in 

 silicates, and this process does not necessarily involve subtraction of any 

 material. Since it involves addition of oxygen, the result is to produce an 

 increase in volume provided the separated silica or a considerable part of 

 it remains in situ. 



As is well known, and as has been pointed out heretofore, the process 

 of oxidation involves great liberation of heat; but the data are not at hand 

 by which the amount of heat liberated can be determined. Concluding in 

 reference to volume and heat relations, it may be said that the process of 

 oxidation perhaps illustrates better than any other the principle that in the 

 zone of katamorjdnsm combinations which take place with the liberation of 

 heat control the reactions rather than the volume relations. Heat is 

 liberated in all the cases of oxidation, and whether there is an increase or 

 decrease of volume is a subordinate factor. 



Where the amount of oxygen is sufficient to cause the iron compounds 

 to be transformed to ferric oxide, there are red or yellow soils and sub- 

 soils. Excellent illustrations of such regions are the southern Appalachians 

 and the Piedmont Plateau, where the crystalline rocks have deeply 

 weathered. As shown by Russell, the red color of this soil is due to a 

 ferruginous clay containing both ferric oxide and alumina, which incrusts 

 the grains of the rocks." 1 The ferric oxide is produced during the decom- 



" Russell, I. C, Subaerial decay of rocks, and origin of the red color of certain formations: Bull. 

 U. S. Geol. Survey No. 52, 1889, pp. 14-15. 



