I5O PRINCIPLES OF AGRICULTURAL CHEMISTRY 



than soils formed from rocks in place, and not mixed with the 

 products of the decomposition of other rocks. 



Primary Minerals. Dr. F. W. Clarke, 1 Chemist to the U. S. 

 Geological Survey, has calculated the relative abundance of the 

 minerals of igneous rocks to be as follows : 



Per cent. 



Feldspars 59 5 



Hornblende and pyroxenes 16.8 



Quartz 1 2.0 



Biotite 3.8 



Titanium minerals 1.5 



Apatite o. 6 



Less frequent minerals 5.8 



Quartz is crystallized silicon dioxide (SiCX) and is widely dis- 

 tributed in nature. It is insoluble in water or any acid except 

 hydrofluoric. It is very hard and not easily broken. It cannot 

 be dissolved or decomposed by natural agencies, although it may 

 be reduced to a fine powder. It is often found as pebbles, sand, 

 and sometimes a very fine powder,' in residues from rocks which 

 have otherwise undergone serious changes. It is of common 

 occurrence in soils. It has no value to plants as food. 



Felspars are double silicates of alumina with potash, soda, or 

 lime. They are widely distributed, making up about sixty per 

 cent, of the average igneous rock. The chief varieties of felspar 

 are: 



Orthoclase, a potash felspar KAlSi 3 O 8 



Albite, a soda felspar NaAlSi 3 O 8 



Anorthite, a lime felspar CaAl 2 Si.jO 8 



A number of intermediate varieties occur, such as oligoclase, a 

 soda-lime felspar. 



Felspars are not acted upon by strong acids (except hydro- 

 fluoric) and can only be brought into solution after fusion with 

 carbonate of soda, or by decomposition with hydrofluoric acid. 

 They are slowly decomposed by weathering agencies. If finely 

 ground felspar is brought in contact with water containing 

 phenolphthalein, the liquid assumes a red color. This is due to 

 the solution of a small amount of soda and potash, which, being 

 1 Bulletin 419, p. 9. 



