CLASSIFICATION OF MINERALS. 



Elements of the earth's crust. K 



193 



From this table it is seen that of the metallic elements aluminum, iron, 

 magnesium, calcium, sodium, and potassium are the only ones which may 

 be called abundant, and that of the nonmetallic elements oxyg*en and 

 silicon are the only two which are abundant, although carbon, sulphur, and 

 chlorine are very important, and still others of the nonmetallic elements, 

 such as fluorine and phosphorus, are of considerable consequence. These 

 elements combined, or rarely alone, as they occur in the natural state, are 

 called minerals. 



The more important minerals are classified into (1) elements, (2) oxides, 

 (3) salts of the binary acids, and (4) salts of the ternary acids. Of the 

 twenty-one elements above given, only oxygen, iron, nickel, sulphur, and 

 carbon occur in the elemental form, and with the exception of oxygen the 

 amounts thus occurring are insignificant. The free oxygen is mainly con- 

 tained in the atmosphere, but large quantities are also included in the 

 hydrosphere and lithosphere. The oxides comprise both hydrous and 

 anhydrous minerals. Of the salts of the binary acids, the sulphides are 

 of predominant importance, but the chlorides and fluorides are of some 

 consequence. The salts of the ternary acids are the predominant minerals 

 of the earth's crust. They include silicates, carbonates, titanates, phos- 

 phates, and sulphates. These compounds are mentioned in the order of 

 their importance; indeed, the silicates are of dominating importance, but 



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