MATERIALS OF THE EARTH 33 



cally. Not rarely they contain as many as three or four basic 

 elements, in union with oxygen and silicon. It is well known 

 that substances which are complex chemically, are, as a rule, less 

 stable than those of simple constitution. Complex silicates, such 

 as the feldspars, the micas, the amphiboles, and the pyroxenes 

 (p. 80) tend to break up into simpler substances. The chemical 

 changes are helped along by the oxygen, the carbon dioxide (CO 2 ) , 

 and the water vapor of the air, as well as by water after it is pre- 

 cipitated. Some of the simpler changes may be noted. 



Oxygen may enter into combination with the iron of a silicate 

 mineral which contains iron. The iron is thus taken out of its 

 silicate combination, and in union with the oxygen forms iron 

 oxide, a simple and stable ^chemical compound. This process is 

 oxidation. Oxidation affects other elements also. 



Similarly the carbonic dioxide of the air may enter into com- 

 bination with the base of a silicate mineral. Thus it enters into 

 combination with the calcium of a mineral which contains calcium, 

 taking the latter out of its combination with silica. The union of 

 the calcium and the carbon dioxide gives rise to calcium carbonate. 

 Magnesium and iron may be taken out in the same way, forming 

 magnesium carbonate and iron carbonate, respectively. This pro- 

 cess is called carbonation and the carbonates thus formed are simple 

 and stable in composition. The carbonates are more soluble than 

 most other common mineral substances. 



Water may enter into combination with mineral matter, and 

 the union is hydration. Thus when iron rusts (oxidizes), it is not 

 merely oxygen which enters into combination with the iron, but 

 water also. Iron rust is a hydrated oxide of iron. 



The changes outlined above may be illustrated by the changes 

 which take place in the decomposition of a complex mineral, such 

 as augite, the composition of which is represented by the formula 

 2CaO.2MgO.2FeO.Al 2 O 3 .Fe 2 3 .6SiO 2 . If to this be added 6CO 2 and 

 2H,O, the products, after carbonation and hydration might be ex- 

 pressed thus: 2CaO. 2MgO. 2FeO.Al 2 O 3 . Fe 2 O 3 .6Si0 2 +6C0 2 + 2H 2 O 

 = 2CaCO 3 + 2MgCO 3 + 2H 2 O . Al 2 O 3 .2Si0 2 + 2FeCO 3 + Fe 2 O 3 + S4iO 2 . 



Oxidation, carbonation, and hydration, involving respectively 

 the addition of oxygen, carbon dioxide, and water, increase the 



