204 A TREATISE ON METAMORPHISM. 



magnesite into hornblende and calcite; or the reverse of this, hornblende 

 and calcite into augite, siderite, and magnesite. 



The changes here belonging are by far the most numerous and impor- 

 tant of the various classes; indeed, are vastly more important than all of 

 the other classes together. By far the greater number of reactions written 

 out on the succeeding pages for the alterations of the various minerals fall 

 under this heading. 



The more important of these alterations, considered from the point of 

 view of the nonmetallic elements, may be classified into: 



(I) Oxidation. (2) Deoxidation. 

 (3) Hydration. (4) Deh ydration. 

 (5) Carbonation. (6) Decarbonation. 

 (7) Silication. (8) Desilication. 

 (9) Siliciiication. (10) Desilicification. 



Less important reactions are: 



(II) Sulphidation. (12) Desulphidation. 

 (13) Sulphation. (14) Desidphation. 

 (15) Titanation. (16) Detitanation. 

 (17) Phosphation. (18) Dephosphation. 

 (19) Chloridation. (20) Dechloridation. 

 (21) Fluoridation. (22) Defluoridation. 



■ (23) Boration. (24) Deborstion. 



A number of these reactions are of small consequence so far as the 

 alterations of rocks are concerned ; but all are important with reference to 

 the development of minerals, and especially in reference to economic 

 products. This phase of the subject in reference to the metallic products 

 is treated in Chapter XII. 



(1) Oxidation is the addition of oxygen. Frequently the added oxygen 

 is substituted for another element, often sulphur. 



(2) Deoxidation is the subtraction of oxygen. Often the subtracted 

 oxygen is replaced by another element — for instance, sulphur. 



(3) Hydration is the addition of water, producing hydroxides. 



(4) Dehydration is the subtraction of water from hydroxides. When 

 earned to completion, anhydrous compounds are formed. 



(5) Carbonation is the union of carbonic acid and base, or the substi- 

 tution of carbonic acid for another combined acid, in either case producing 

 carbonates. The oxide with which carbonic acid most frequently unites is 



