264 MATERIALS AND THEIR ARRANGEMENT 



combination with the calcium of a mineral which contains calcium, 

 taking the latter out of its union 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. proc- 

 ess is called carbonation, and the carbonates thus formed are simple 



Fig. 241. Exfoliation of granite. Wichita Mountains, Okla. 



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 (see limonite, p. 256). 



Oxidation, carbonation, and hydration, involving respectively 

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

 volume of the mineral matter. The result is that the rock affected 

 crumbles. Thus the iron rust formed on a knife blade crumbles 

 off. So the iron rust formed when oxygen and water unite with 

 the iron in the rock, causes the rock in which the change takes 

 place to crumble, partly because of the expansion involved. 



Again, some of the simple compounds, especially the carbonates, 

 formed when the rock decays, are somewhat soluble and may be 

 dissolved and taken away. This tends to make the rock less com- 

 pact by taking away one of its ingredients. 



Oxidation, carbonation, and hydration therefore not only 



