RELATION OF HYDRATION AND DEHYDRATION. 483 



earned on in a laboratory, dehydration is very appreciable." These tem- 

 peratures are not reached under natural conditions, but in hot reg-ions 

 where water is not plentiful it is probable that partial dehydration of 

 such compounds as the zeolites, colloidal silicic acid, etc., takes place on a 

 considerable scale. Doubtless the process in reference to these minerals 

 has not been commented upon because ordinarily there is no accompany- 

 ing change in color. To illustrate, the table on pages 375-394 shows that 

 aluminum hydroxide, as gibbsite and diaspore, forms on a vastly greater 

 scale in the belt of weathering than ferric hydroxide. It is highly proba- 

 ble that, under the same conditions in which dehydration of iron occurs, 

 dehydration of aluminum hydroxide also takes place to some extent. The 

 same statements may be made in reference to the other hydrous minerals 

 which lose their water or a part of it at a low temperature, as, for instance, 

 the zeolites. 



It has already been stated that hydration is the most extensive 

 reaction of the belt of weathering. This is at once found by reference 

 to the classified tables of alterations, page 402. It is there seen that a 

 number of the important oxides and many of the more important silicates 

 may be altered by simple hydration. The process of hydration, as 

 explained in Chapter IV, from a physical-chemical point of view, involves 

 expansion of volume and the liberation of heat. The amount of heat 

 liberated is great, as shown in Chapter V. Whether hydration occurs 

 alone or occurs combined with the other processes, there is an increase in 

 volume. In simple hydration the volume increase ranges from a very 

 small per cent to as high as 160 per cent, as in the alteration of corundum 

 to gibbsite. Commonly the increase in volume is less than 50 per cent. 

 The quantity of heat liberated in the process of hydration is great, but 

 the average amount can not be quantitatively stated. 



OXIDATIOX, CAKBOXATIO.N, AXD HTDKATIOX. 



As already noted, oxidation, carbonation, and hydration may each 

 take place separately, but commonly two or three of these processes are 

 simultaneously at work on the same rocks, and even on the individual 

 mineral particles. No average can be given as to the amount of increase 



f'Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. IT. S. Geol. Survey, vol. 43, 

 1903, p. 262. 



