6 CHEMICAL ELEMENTS CONSTITUTING THE EOCKS 



Calcium is a very important element of the earth's crust, 

 although it has been estimated to compose only about 3.5% of 

 its mass. Its most conspicuous form of occurrence is in com- 

 bination with carbon dioxide, forming the mineral calcite 

 (CaCOg), or the rock limestone. In this form it is slightly 

 soluble in water containing carbonic acid, and hence has be- 

 come an almost universal ingredient of all natural waters, 

 whence it furnishes the lime necessary for the formation of 

 shells and skeletons of the yarious tribes of moUusca and corals. 

 In combination with sulphuric acid, calcium forms the rock 

 gypsum. It is also an important constituent of many silicates. 



Magnesium is found in combination with carbonic acid as 

 carbonate, forming thus an essential part of the mineral mag- 

 nesite and the rock dolomite. The bitter taste of sea-water and 

 some mineral waters is due to the presence of salts of magnesia. 

 In combination with silica it forms an essential part of such 

 rocks as serpentine, soapstone, and talc. 



Potassium combined with silica is an important element in 

 many mineral silicates, as orthoclase, leucite, and nepheline. 

 In smaller amount it is found in silicates of the mica, amphi- 

 bole, and pyroxene groups. The following table will serve to 

 show the varying amounts of potash (K2O) in rocks of various 

 jakincis z 



Granite 2.6 to 6.50% 



Diorite 0.1 to 2.42% 



Basalt 0.058 to 0.50% 



Gabhro 0.00 to 0.93% 



Limestone 0.19 to 1.22% 



Sandstone 0.00 to 3.30% 



Slate (fissile argmite) 0.00 to 3.83% 



As a chloride, potassium is invariably present in sea-water, 

 and as a nitrate it forms the mineral nitre, or saltpetre. 



Sodium, — The most common and wide-spread form of the 

 element sodium is the compound with chlorine known as sodium 

 chloride (NaCl) or common salt. In this form it is the most 

 abundant of the salts occurring in sea-water, and constitutes 

 also rock masses of no inconsiderable dimensions interstratified 

 with other rocks of the earth's crust. Combined with silica, 

 lime, and alumina, sodium is an important constituent of the 

 soda-lime feldspars, and of numerous other silicate minerals. 



