20 PHYSICAL GEOLOGY 



magnesia (MgO), soda (Na 2 O), and potash (K 2 0). The union 

 of silica with the other oxides named, forms silicates. In 

 similar manner, very many other combinations of elements, 

 both simple and complex, occur in nature. The few elements 

 which exist free as constituents of rocks, together with many 

 definite compounds of elements which naturally take the solid 

 form, are minerals. Ice is truly a mineral in this sense, but is 

 not popularly so considered. Thus combinations of elements 

 give rise to minerals, and aggregations of minerals form 

 rocks. 



When studied by themselves in larger pieces, minerals are 

 found to have certain definite characteristics in addition to 

 their nearly constant chemical composition. When formed 

 under favorable conditions, most of them assume geometrical 

 forms that are constant for each mineral. Common salt, 

 for example, forms cubes. The form of any given crystal is 

 determined by its internal structure, probably by the ar- 

 rangement of the particles (molecules) of which it is com- 

 posed. Even though the external form be marred or de- 

 stroyed, this internal crystalline structure remains the same 

 for each kind of mineral, and may be observed with the aid 

 of a microscope in even the most irregular pieces of the sub- 

 stance. 



Other convenient differences which may be used to tell one 

 mineral from another are found in their color, manner of 

 breaking, hardness, and luster. Some minerals break with 

 a shelly fracture, like glass; others leave ragged surfaces, 

 while many split more or less perfectly along certain planes, 

 and thus leave shiny, flat surfaces. This relatively easy 

 splitting in certain planes is called cleavage. It is one of the 

 most distinctive features of many minerals. 



Of the more than 800 varieties of minerals that have been 

 named and classified, fewer than 50 are important either 

 geologically or commercially, while about 8 make nearly all 

 of the common rocks. Most of the metals are derived from 

 a few more. 



