34 ELEMENTABY AGKICULTtJRAL CHEMISTRY 



ture and composition, are called minerals. Distinctly separate 

 minerals are most frequently to be found in the igneous rocks. 



Minerals and Rocks. — The following minerals are ex- 

 ceedingly abundant, and are of importance in agriculture : 



Quartz is chemically the oxide of silicon, SiOg. It has 

 been estimated that 35 per cent, of the solid crust of the earth 

 is composed of quartz. It is one of the hardest and most 

 durable of substances, being almost insoluble in water and 

 little affected by weather. In many cases, however, the other 

 constituents of rocks are acted upon by atmospheric agencies, 

 and the quartz crystals, being thus loosened, are removed by 

 running water. Fragments of quartz consisting of crystals 

 rounded and worn by mechanically rubbing against each other 

 form the largest constituent of many soils. Such sand is 

 devoid of plant food. 



Felspar is probably the most abundant of all minerals, con- 

 stituting, it is estimated, 48 per cent, of the earth's crust. 

 Chemically felspar is a double silicate of alumina and potash, 

 soda, or lime. The chief varieties of felspar are ; 



Orthoclase, KjO.Al^Og.eSiO, ; 

 Albite, NajO.AlgOj.eSiOa; and 

 Lahradorite (Na2.Ca)O.Al,03.3SiO,. 



Orthoclase, or potash felspar, is the most important. It is a 

 hard mineral, often coloured pink or green, though sometimes 

 tvhite. Although hard, it is easily attacked by water and 

 carbon dioxide, the potash being largely removed in solution 

 as carbonate and silicate, while the final residue left is kaolin, 

 or china clay, Al303.2SiOj.2IIjO. Orthoclase furnishes a con- 

 siderable quantity of the potash found in soils. 



Mica, another abundant mineral, characterised by its 

 tendency to split into thin elastic plates, is essentially a 

 wlicate of alumina and potash, 3Alj03.K30.4SiOj, though 



